The Lost World: Houyhnhnms versus Vogoraks. (What? You don't know what Vogoraks are? Read on... read on...)
Once the PCs get far away from civilization, they encounter a beautiful waterfall in a lonely mountain pass region that looks like humans have never set foot in before. Someone notices that the waterfalls are glinting more brightly than one would usually expect -- like there's something bright behind the falls.
Of course it's an entrance into a lost world. Depending on the size, feel free to place D&D's anachronistic dinosaurs here (or freakify them a bit), but the true masters of the land are intelligent, civilized horses. The glinting light is their city, which is just over the horizon, maybe an hour's walk away from the waterfalls. Whether or not the horses call themselves Houyhnhnms depends on whether or not the players have read Swift's Gulliver's Travels!
The horses wear clothing, speak the PCs' common tongue fluently, and have all the accoutrements of civilization that are standard in the human world that the PCs come from. There are horse fighters, clerics, magic-users, and thieves, as well as "normal" horses, NPC sages, specialists, and so on. Horse blacksmiths are needed to make horseshoes, of course! :-) Don't forget the rare "demi-horses" (unicorns, pegasi, hippogriffs) on the fringes of society that remind the PCs of elves, dwarfs, etc.
When a small group of Houyhnhnms first encounters the PCs, they will first try to talk with the PCs' horses (if they bring mounts past the waterfalls) to see what's up with the strange two-legged creatures they have captured. They'll be shocked when they figure out that these visiting horses cannot speak. They will be doubly shocked to see who's in charge -- and then triply shocked to see humans wearing clothes and having the ability to speak! The PCs will hear murmurs such as "Oh, my... these creatures do not look like any Yahoos that I've ever seen." and "They look incomplete, somehow, but I cannot put my hooves on exactly how."
If there are horse fighters or soldiers among the first ones to encounter the PCs, they will attempt to arrest the humans and haul them back to the city. Houyhnhnm fighters of levels 1-2 attack as "medium" warhorses (1-6/1-6/1-3 hoof/hoof/bite), at levels 3-5 they attack as "heavy" warhorses (1-8/1-8-1/3 hoof/hoof/bite), and at level 6 they gain an extra attack every other round. If wearing a unicorn-spiked helmet they can charge as a unicorn for 2-24 hp of head-butting damage. Some of the most experienced fighters will have magic +1 horseshoes.
If the PCs are arrested, the authorities will take their time to mull over what to do with these odd creatures. The longer the PCs stay in the area, the more likely they will hear about the threat of attack by the fierce beasts known as Yahoos, who have been raiding the outskirts of their "civilized" lands and are coming ever closer to their peaceful city. The Houyhnhnms are divided about how to deal with these creatures. Some don't want to wait for the Yahoos to invade, and they advocate searching out their dens and exterminating them. Some are beginning to plan new defenses to the city. Others just want to be left alone and deny there is a threat at all.
However, on the morning of the second full day the PCs are in the Houyhnhnm city, the attack comes. A semi-organized band of 170 Yahoos comes down out of the hills to the north of the city and begins to plow through the city.
Readers of Swift will think the Yahoos are just primitive, caveman-type humans. Sorry, nope. Meet the Vogorak:
|Click to enlarge.|
On the other side of the ledger, the full population of the Houyhnhnm city is 1500. Of that, 600 are young foals, ponies, and the elderly. That leaves 450 adult stallions and 450 adult mares to defend the place. The females and non-fighter males fight as wild horses (single 1-3 hp attack). The local militia is comprised of 80 fighters, with a range of levels between 1 and 6. There are 10 horse magic-users throughout the city (though half of them will flee), and there are 50 horse clerics (of whom only 10 will flee). It doesn't look good.
Nobody knows why the Vogoraks are attacking, by the way. Maybe they need to expand their territory for food. Maybe they just can't stand the whinnying of horses.... Nobody speaks their language.
Anyway, if the relations between the Houyhnhnms and the PCs hasn't been too bad, they will beg for the PCs to help them defend their home, and will promise untold riches if they succeed in aiding them to repel the invaders. (Even though they do have gold, the PCs may be disappointed to find that any magic items they will give as gifts will work only for equine operators...)
If you don't have a good way to do skirmish-level mass combat, try Zzarchov's system, or Talysman's system, or Chris' Birthright-inspired system. See also this entry in an old online book of RPG plots. There's lots of food for thought, along with examples, in a series of relevant posts by Alexis.
Full disclosure: The Vogorak is not my own personal creation. The above sheet was created by one of my high-school compatriots from an epic 1980s AD&D campaign of yore. Let's call my friend War Machine. I'm reasonably sure that Mr. Machine has left behind all interest in tabletop RPGs and probably no longer even has the original sheet (the above was scanned from an old photocopy). What I am 100% sure of, however, is that he would be delighted -- though, possibly, slightly embarrassed -- if his old creation was found useful by some modern-day OSR grognards. I'm no lawyer, though, so if you like it, re-skin it. :-)