Real Life Military Units

1-Forlorn Hope

Not to be confused with penal units, forlorn hopes are made up completely of volunteer soldiers who have been sent on extremely dangerous missions with the promise not only of medals and extra pay but of being promoted one rank up if they survive and succeed in their mission. In armies where only the nobility can normally become commissioned officers, this gives NCOs of non-noble birth the chance to rise to junior officer rank in the army, perhaps even becoming knighted as "hedge knights." Historically, forlorn hopes were quite common in the Napoleonic wars.

2-Penal Units

Penal units are composed either of soldiers who have broken military law in some way or even of outright convicted criminals taken from prisons and offered their freedom once the war is over if they survive. Historically, the Nazis had the Battalion Dirliwanger which was composed of poachers, hardened criminals, political prisoners and even a few men from insane asylums who committed horrifying war crimes, and the Soviets had their strafbats of disgraced soldiers and Gulag inmates. Later, Putin would hire convicts to fight in Ukraine in exchange for their freedom. Like forlorn hopes, these units are sent to the most dangerous areas, but unlike forlorn hopes they get no rewards except perhaps being sent to regular military units.

3-Barrier Troops

Barrier troops are placed behind the other regiments of the army to capture or kill deserters and prevent any unauthorized retreats by opening fire on their own soldiers if necessary, and are often placed behind penal units to stop them from running away. Stalin's NKVD provided barrier troops in World War Two.

4-Special Forces

The best of the best, these soldiers are the best armed, best trained, best fed soldiers in the entire army, and are often used on secret missions. They are normally volunteers. King Alexander the Great had his Companion Cavalry, Hitler had his  Liebstandarte SS, the British have the SAS and the Americans have their Green Berets and SEAL teams.

5-Elite Troops

Not quite the best of the best, these are the regiments whose soldiers have a chance of getting into the special forces if they want to. They are well armed, well trained, with high morale and regimental pride. Units such as the US Marines and Coldstream Guards would be cases of real life elite troops.

6-Conscript Troops

These soldiers don't normally want to be in the armed forces, and often suffer from low morale, brutal hazing, bullying NCOs and other such problems, but can be good troops if they are trained well enough. If involved in a long grinding war with no end in sight, they have been known to murder their officers if they think they are incompetent glory hunters.

7-Milita 

Militia are non professional soldiers who are used in times of war as skirmishing troops to slow the advance of the enemy's  regular soldiers until their own regular soldiers can  show up and deal with the problem.

8-Knights

Heavily armed and armoured warriors often capable of cutting through weaker opposing troops, these had several major disadvantages historically. They were very expensive to hire and provision, often only owed service of forty days per year and could avoid even that if they paid a tax called scutage, were impetuous and hard to command and control, and preferred to capture each other for fat ransoms rather then kill each other if possible. Massed longbowmen could riddle them with arrows that could penetrate even their plate armour.

9-Light Cavalry

These are used for tasks such as scouting and pursuing fleeing soldiers rather then carrying out massed charges and wear little or no armour and are very useful in those roles, but are not suited for a full boot to boot cavalry charge against hardened troops of high morale.

10-Bombadiers 

These troops are in charge of the artillery of the army, that depending on the time period can range from trebuchets or light cannons to heavy artillery capable of firing huge shells for many miles or even nuclear, biological and chemical weapons.

11-Gurrilla Forces 

Acting without uniforms, popping up for ambushes or assassinations and then melting away into the general population, these are hated by regular troops, who are likely to shoot them on the spot and/or carry out atrocities and war crimes against the civilians-which rapidly loses them support and/or creates more foes to fight. The Provisional IRA or the Vietcong would count as such soldiers, as would the ninjas of Japan. 

12-Mercenaries

They fight not for their country or for a cause but strictly for pay and for pay alone, and may legally leave if they are not being paid properly. Attitudes towards them vary wildly, from being seen as no worse then ordinary soldiers to being hated so much that they are slain rather then being taken prisoner, and their own professionalism and obedience varies widely from committing war crimes at every opportunity all the way to behaving better then regular troops.

13-Armed Civilians

In countries with little or no gun/weapon control, these can be a short term menace, but they lack the skills of trained troops and can't generally hold their ground for very long. Those who survive often turn into Guerrilla Forces, those who don't end up dead or captured very quickly or flee and join the regular armed forces of their country, or just give up the struggle altogether. 

14-Foreign Legion

These serve another country as regular forces. The French Foreign Legion and the Vatican Papal Guards are two examples from real life. Discipline in such forces is often very strict indeed, but morale is high and training and equipment are both good.

15-Clan Warriors

The Highland Clans who fought for Bonnie Prince Charlie would count as clan warriors. They tend to have relatively few tactics apart from ambushes and charges, but are fearsome and formidable in attack, although their discipline often leaves much to be desired.

16-Klan Warriors

If they are fighting soldiers that they consider to be of their own race, they are often no more likely to commit war crimes then any other soldiers, but they are merciless towards those who they consider to be of an inferior race. The Confederates at the Battle of Fort Pillow, the SS, and the US soldiers who fought against the Japanese in World War Two would count as Klan troops. 

17-MacNamara's Morons

Robert MacNamara historically sent people who were mentally challenged to fight in Vietnam. Whilst a few proved to be good soldiers and at least one earned a promotion to sergeant, most were utterly useless as soldiers and a danger to themselves and their comrades.

18-Child Soldiers

It may be a war crime to use them, but children often make very good soldiers. They can be conditioned to obey orders without question, if you shoot some of them who are out of uniform and the weapons are removed successfully by their comrades it can end up looking like you committed a war crime, and individual child soldiers out of uniform can get up close pretending to be a terrified refugee and then open fire with a hidden pistol. 

19-Rusty Troops

They were properly trained in basic training, but since then have not been trained for a long while either because their country lacks the money for it, or has never been to war since it's foundation and does not see the need for it. In effect, they are at the level of militia, able to skirmish and better then just armed civilians but not by much.

20-Fort Defenders

They have spent their entire military service so far defending forts and walls and are very effective at that, but not quite so effective on an open battlefield as that has not been what they were trained for, and soldiers do things well that they have been properly trained to do. Their job in war is to remain on the defensive until the rest of their army can come to their aid. 

21-Paratroopers

These hardened soldiers have been trained to parachute into enemy territory and are often special forces or close in skill and training level to them, with good morale and a will to win, and are incredibly good at their job.

Fantasy Troops

22-Undead

These are a mixed bag depending on what kind of undead they are. At the bottom level are shambling zombies and animated skeletons that have been animated by a necromancer. Whilst they don't have morale problems like military units of the living and are scary to look at, they are quite easily dealt with. Bone splinters easily when struck with weapons, and the zombies are almost as easy to kill. Then there are undead such as Ironbones that have been plated with metal, and Greater Zombies that need a headshot or to be beheaded to kill them and are much more dangerous. Then you have the Greater Undead like ghouls, vampires, and liches-whilst likely to be few in number they need magical weapons to despatch, but they can suffer from low morale if hard pressed and unlike the lowest undead, will not press their attacks until destroyed.

23-Orcs

Depending on your world, Orcs can be clan warriors, serve as mercenaries, be well trained troops., knights, penal troops, or any kind of troops from this list. There is no need to just have them as undisciplined clan warriors. Certainly, whatever kind of troops they are, they likely have a reputation as ferocious fighters.

24-Goblins

Goblins are more likely to be used as skirmishers or scouts, or at the controls of artillery, as they are weaker then Orcs and most likely weaker on a one-to-one basis then humans as well. 

25-Elves

The classic Elven unit that first comes to mind is an archer unit, but again, like with Orcs they could be almost any of these units, so don't be afraid to think outside the box.

26-Mages

Mages are expensive and highly skilled, so only the largest of your world's armies will have entire platoons of them, but their magic more then makes up for their lack of numbers. They might be weather mages, necromancers, fireball casters or other types, and one or two skilled mages will be a major foe for the PCs.

27-Robots

Be they non sapient drones, droids or mechs or fully sapient robots, with their armour and blasters and the fact that sapient robots can, unless jammed electronically, mind transfer to spare bodies if they have to, robot troops are going to be some of the hardest fighters that the PCs will ever  face. If fully sapient, they will likely have a hatred of biologicals if they had bad experiences with them in the past.

28-Flying Troops

Be they mounted on eagles, pterodactyls, dragons, or strange flying machines, they can scout long distances and carry out airstrikes on the foe with bottles of napalm, crossbows and swords, but if mounted on living creatures, their mounts will not swoop down on those armed with long spears or firearms as they don't want to die.

29-Stone Men

Stone-skinned trolls, all but the sharpest swords will only do minor damage, but war-hammers and maces do them much more damage and a critical hit on one of their limbs with such a weapon can sever it. Perhaps they can only come out as night as sunlight turns them fully to stone, or perhaps not, depending on your world. They make good shock troops.

30-King's Guards

These are the classic special forces of the fantasy world, the warband and bodyguards of the King himself, and should be treated as such. Their weapons, armour, morale, food and training will all be first rate.