Updated with Gorgons and Wendigos.
Also for this update are Kamaitachi (flying weasels with anesthetic venom slashes), Qilin, Loong, Feathered Serpents, and Griffons.
Adamant Lions are another semimegabeast based on the Nemean Lion. Its skin and claws are as hard as adamantine. Its skin is still flexible, though, so blunt damage will work. Its leather has the same properties.
Golden Deer are very rare, fast creatures, sacred to the gods of the hunt. If you see one, chase it - if you keep up the chase and get close enough, you have a chance of gaining a blessing of enhanced speed that manifests the next day. HOWEVER, don't actually wound it or kill it, or you have a chance of being cursed with bad luck (like a mummy's curse).
Centaurs are now in and they are playable. Their personalities are particularly interesting, being a mix of cultured and immoderate. They are alcohol dependent and will raid fortresses for alcohol, but they also make good teachers when they do join civilization. They are good runners and fighters thanks to their large size and hooves. Not such good climbers, though.
Kappas have been modified, they are smaller now (about the size of children) but strong enough to wrestle with the big races. When their head-plate is wet, they gain almost all vampire perks. I found that you can also refill your head by spilling your waterskin on the ground and crawling through the puddle, which makes sense; although the infinite water exploit may allow you to abuse this. They seem to have problems showing up in worldgen, though; you can increase the chances of playable kappas by increasing the savagery of your world (which isn't a bad idea if you want to encounter this mod's creatures in general).