Rockodile - A crocodilian with a stony hide. A very tough armor can be made by tanning its hide.
Bugorse - A large bug used to replace the now extinct horse.
Labor Lizard - A large lizard used as a pack animal.
Siege Tortoise - A horse sized tortoise that destroys buildings and tries to steal food. It might have a tougher than usual shell or skin, I want it to be challenging.
Blood Bat - An eagle sized bat that drinks blood. If possible, I want to hunt dwarves for blood. I've noticed that giant mosquitoes don't really do that.
Kelenken -
A real but thankfully extinct animal. A chocobo-like bird with a skull the size of a human torso and a giant hooked beak. According to some estimates, the Kelenken and its relatives, the Terror Birds, could run as fast as lions and cheetahs, possibly faster, and as birds, they would not get tired as easily. Other terror birds might join the Kelenken if I need more biodiversity. Butchering a Kelenken allows you to use its beak to make special weapons.
Sebecus -
A real but thankfully extinct animal. A crocodilian adapted for life on land. Their legs are long for running, and their heads and teeth are shaped differently. A modern crocodile has a flat head and peg like teeth, designed to grab animals and hold on, so that the crocodile can pull them into the water and drown them. The Sebecus' head is very tall and narrow and its teeth have serrations like a shark, designed to give its bites the ability to crush bone and tear flesh in one shot. The Sebecus is still a mostly unknown fossil so its speed is unknown, but modern alligators can run on land at about 10 mph and the Australian perentie can run 25 mph. Both crocodilians and monitor lizards are known for their ability to accelerate very quickly.
Hellion Hog -
A real but thankfully extinct animal. Known more frequently as terminator-pigs or Entelodonts. A massive pig-like creature with long legs adapted for running and an enormous jaw for devouring prey. The largest are as large as horses, and can be trained as mounts.
Thunder Beast -
A real but thankfully extinct animal. An entire class of extinct creatures known as Brontotheres, which means Thunder Beasts. They are like rhinos, but with bony paddles instead of horns, and some are much bigger.
Gravel Beast -
A real but thankfully extinct animal. The head of a horse, the body of a gorilla and giant claws on the hands. That really happened. I might use the specific species Moropus instead of the family name Chalicothere (Gravel Beast) as it sounds a little nicer, however the moropus is more normal looking than other Chalicotheres.
Oliphaunt - Those giant elephants from LotR.
Land Squid - A squid that lives on land. 8 feet, 2 arms, a beak and ink spray. It would be cool to give them the ability to jump by blowing air out of their jet organ, but I don't think DF is equipped for that kind of locomotion. There might be specific kinds like a Tree Squid, Rock Squid, Sand Squid or River Squid.
Spit Viper - A poisonous snake that spits its venom.
Blitztang - A wild relative of the horse. They are fast, tireless and belch flames when threatened. It would be interesting to have them give some kind of a flame-organ when butchered, but I'm not sure what you would do with it. Maybe it turns into coke, but that seems like to small of a reward. Maybe they have a gut full of coke or fire pearls or something.
Owlbear - You know the one.
Sabear - A bear with saber teeth and saber claws. Sabers can be used for weapons after butchering.
Spear-Tongue Toad - A toad with a piercing tongue attack. Touching the skin causes intoxication effects, including items made from Spear-Tongue Toad leather.
Killeroo - A killer kangaroo. Maybe with saberteeth. Maybe with more killery foot claws.
Killer Ape - I'd like to make something like a super gorilla or barsoomian White Ape, but I'm not sure about the details yet.
Sharkasaur - A shark-therapod hybrid, like the pokemon garchomp or Disgaea's sea demons.
Scaled Wolf - A wolf-sized lizard that hunts in packs. Maybe tamable into a Scaled Dog.
Marrow Walker - A thin creature with an exoskeleton that looks like bone. Not sure yet if it should be biped, quadruped, predator, grazer or what.
Wolf Spider - A giant spider with the face of a wolf.
Mountain Climber - An 8 legged beaked reptilian that lives in mountains and is great at climbing.
Ramblejaw - A dog sized bipedal reptilian creature with a head that is as large as its body.
Bladebug - Something like the pokemon Scyther or D&D's swordwing. Butchering a bladebug will allow you to use its bugblades to make special weapons.
Cryptids - I might use a few "real-life" cryptids too, like Burrunjor, Nessie, Jackalope, etc. Name your favorite cryptids.
Pseudodragon - Cat sized or dog sized dragon-like creatures. I will probably use a different name to avoid the D&D creature.
Dune Worm - You know the one. Various sizes, the largest ones would be the rarest, and possibly megabeasts. You might get something spicy for butchering them.
Macetail - Some kind of glyptodon/ankylosaurid animal. The mace-like tail can be turned into a weapon.
Scorpileo - A lion-like animal with a scorpion's tail. It is not poisonous, but can impale its victims with terrible ease. The tail can be used to make weapons after butchering the Scorpileo.
Hoofbird - An ostrich like bird with hooves and antlers. They have beautiful tail feathers that can be acquired for decoration. Shearing or Butchering.
Asambosam - An african vampire/ogre. Sasambosam have hooks for feet and iron teeth, so they hide in tree branches and either drop down on victims or grab them and pull them up, depending on the story. They sound to me like a sloth, but are said to look like people. Butchering one (if it's not person-like enough) would allow you to use its iron teeth and hooks to make weapons. I think that these would not actually be made of iron, just an iron-like tooth/claw material, so that you cannot melt them down. They might have iron-like bones as well. Something should have iron bones.
Puck - A mischievous spirit. I would love to make them like the Brownies in Aterlier Iris games, little fairy/dwarves with enormous hammers, or at least like Terry Pratchet's Nac Mac Feegel being very small but insanely strong. They might get civilization status, but it would also be interesting to be able to catch them and raise them as pets or something.
Tentacles - Add tentacles to things.
New Megabeasts - Something more Cthulu- or Godzilla-like instead of the classic mythology fair that we get now. These should also be the source of especially powerful or valuable butcherable items, bones of steel, scales of gold, teeth of diamonds, claws of adamantine, etc. Different species of Chimera would be cool.