Yes - camels and war elephants both seem to come up in topics about cavalry a lot, and as far as I know, Reaper is one of the few (if not only) manufacturers out there making either war elephants (war mammoths, rather) or camels (pack camels, rather). There might be some in metal out there, and I think I've seen some camel cavalry in 1/72 scale, but I'm not really familiar with metal minis anymore, or 1/72 scale soldiers.
Naturally, there's quite a bit more regular horse cavalry out there, but it seems to be hit-or-miss, and wargamers seem to always be looking for more. Horse cavalry seem to be the best bet going, for any cavalry kit-bashing.
After that, there are a few wolf and dog cavalry out there, with the Northstar Oathmark goblin wolf-riders being perhaps one of the better kits I know of for wolf cavalry, and Mantic making most of the dog cavalry I can think of.
Wargames Atlantic have most of the in-print spider cavalry I know of (I'm sure I've seen some in white metal back in the '80s, but I don't think those models are in print anymore.)
There might be some lizard cavalry out there - it seems like a good choice for fantasy gamers. I don't think I can think of much though, except in some Wizkids Pathfinder or D&D prepainted collectible mini sculpts. There might be some Star Wars collectible pre-painted mini cavlary of that sort, too, but I don't know of anything in particular. Wizkids D&D/Pathfinder miniatures also include the odd wolf-rider goblins, dog-rider halflings, and goblin-dog rider goblins. I've also seen one salamander-riding goblin in D&D/Pathfinder minis, and one small manufacturer recently made some frog-riding gnomes.
Skeletal horses seem like a popular request from fantasy gamers, but I don't think I know of any beyond some Wizkids collectible D&D/Pathfinder mini sculpts (or, rather, at least one of these, and I'm not even sure how common it is.) Reaper's made some mountd Bones "not-Ring Wraiths" recently, those looked great, but I think the horses looked like they were living.
There might be some undead cavalry and wolf-riders for LotR gaming, but I'm really not very familiar with those miniatures.
Dragon-riders seem like popular artistic sculpts, but these seem rather too big, heavy, delicate, and expensive for routine gaming use. I'm thinking most of the ones I've ever seen have been in metal and resin, there might have been one or two in Reaper bones as well.
Reaper also produces at least one bear-riding dwarf, and one pony-riding dwarf.
I think there might be some boar-riding dwarfs out there, too.
I can dig the fantasy-kitchen-sink thing, but the halfling dog-riders and dwarf boar-riders seem a bit goofy for my tastes: not quite as fun and adventurous as riding alien creatures, and just a bit too strange and unlikely for straight-laced Tolkienien generic fantasy compared to ponies. For whatever reason though, these cliches seem to have more or less stuck as the fantasy defaults - I'm guessing it comes from Warhammer or Warcraft, but I'm not familiar enough with either to say so for sure!
Short of alien monsters, an alternative that might appeal to me a little more than bear, dog and boar cavalry Lovecraft's Dreamlander zebra-riders (easily improvised by painting regular horses in stripes), with some of the most exotic-looking riders I can find - sure, zebras are about as well-suited for the task as bears, boars, or dogs, but if I'm going to do niche real-life animal cavalry, that seems a little more innocuous, a little more plausible, a little more traditional, and a sight less commonly-used than most.
In the end, though, I think proper war elephants, mules, and camels seem like excellent choices that both fantasy and historical gamers can find uses for, even if only for niche markets, while dinosaur/dragon/lizard-riding cavalry seem to be an under-represented traditional fantasy option.
Now, someone out there did modify a Reaper Bones "Axebeak" prehistoric-megafauna-looking bird with a saddle and rider, and it's a cool conversion - check it out: (link)