Skaven
Rodents of unusual size? I don't believe they exist.
Technologically advanced rat people. Ugly, evil creatures that spread plague wherever they go and topple kingdoms for fun and profit. Due to their chronic backstabbing disorder, nobody trusts them (except Dark Elves, who have a treaty with them), likes them, or wants to be 'allies' with them. Rather bitey. Contrary to popular belief, they are neither American (that would be the Dark Elves) or jailhouse gay.
Clans
They are made up of many clans, the most important ones being:
Clan Pestiliens (Plague monks and guro fetishists)
Clan Eshin (Ninja rats and assassins)
Clan Moulder (Master Mutators)
Clan Skryre (Steampunk rats, they use radioactive arquebuses (though an arquebus is much shorter and less rifled than a jezzail), warp lightning and magitek. Also DOOMWHEELS).
Society
As you may expect (if you do not know much about rat social behavior, which is fairly well developed and includes among other things evidence of rodent altruism, or work on people's stereotypes of what rats are like rather than how rats interact), Skaven society is rather literally cutthroat when it comes to promotions. The only reason why their society has not murdered itself into extinction is because of a very high reproductive rate. Despite their teamkilling tendencies, they obey the Grey Seers, the prophets of their god, The Horned Rat, without question. Except for the other Grey Seers.
Army
Skaven are your standard easily abused horde army. Lots of cheap vermin, whose numbers allow them to easily ignore their one theoretical weakness: shitty leadership, backed up by more expensive and/or specialized units, that are in theory unreliable but will still wreck your shit moar consistently than most anything else by sheer volume. Also, DOOMWHEELS.
Under current rules they are have always been considered overpowered, except for a brief period where DoC reigned thanks to your Spiritual Liege. They have now reclaimed their mantel, since 8th edition heavily favors mass infantry blocks, and the Skaven can easily throw out a block of 100 models for less than what some other armies will spend on a lord, no, I'm not exaggerating, which under the current rules is virtually unbeatable.
Fun Facts
- Grey Seers regularly ride giant bells on scaffolds into battle.
- DOOMWHEELS.
Backstabbing little bastards, they'll fuck you up five different ways without you even knowing about it. If you're lucky.
DIE DIE MANTHINGS!
The above sentence clearly illustrates the quirks of skaven language: they often say certain words twice (words like "die die" and "fool fool" are popular) Also, they often end the name of a species with the suffix -things, so manthings, dwarfthings etc. etc.
Warhammer 40k
While no direct space Skaven exist, there are ratlike mutants described in the fluff, rat-worshiping cultists in Necromunda, a mutant race called Ratlings which despite being more halfling than rat could be argued as a successor, and finally the Imperial Guard itself which serves as uneducated and amoral xenophobes who are mass-bred and treated as currency by the Imperium with access to some nice and fancy toys which are more likely to cause teamkilling than damage to foes. The Hrud used to be the Space Skaven, but that was pretty much retconned. They look different now (on the rare occasion they are mentioned in the fluff), so the only actual space Skaven are the Veer-Myn from Warpath by Mantic games. As usual, most of their models are shitty, but there is great potential for warlock engineer conversions.