Only word legible on that cake is Happy.
Looking at the wiki, [CAN_LEARN] stops butchering, whereas [CAN_SPEAK] makes them eat and drink. Hmm, after some testing with talking sheep, cave ponies are happy to shear and milk them, and will even butcher them, but refuse to make totem out of their skulls, decorate things with their bones, tan their hides, render their fat, and the meat and organs that are produced by slaughtering them don't appear in the kitchen stocks menu. They also all have names. If I just apply [CAN_SPEAK] to the ewe caste, only the ewes have names, and their stuff that implies a skill is unusable, while the rams are cooked up into big tasty stews. So just adding [CAN_SPEAK] seems to solve the basic problem, though the language comes from the civ, so we can't make a language file with cow puns and have only the cows use it, as far as I know.
Making a language would be pretty easy for Dorf Fort. Just look up a word generator on a constructed language website and feed it a softened set of slavic consonants and allowed syllables, have it spit out a few hundred words based on those rules. Any words that look cuter as English portmanteaus can be that, the rest assigned a random word from the list. Makes me wonder how we're going to name the more... delightful concepts like incest. Famsnu, short for family snuggling, maybe? I've been thinking about pony language a little, and reading
Scandinavia and the World, so I've currently got it into my head that the cave pony language should resemble Icelandic. After all, they both live in places with lots of lava and an entrance to the underworld. For the diomedians, I dunno, but we can't really have them speak Nadsat like they do over in Glitterglen if the main pony language is going to be Anglo-Slavic. Maybe make them speak Slavo-English, have them zig where the ponies zag.
Ehm... I just genned a new world and there are beastmen there. Despite the [DOES_NOT_EXIST] tag in c_variations_default.
Beastmen really shouldn't be appearing any more, at all, I've manually removed the entries. Which ones, where and were they wild or part of a civ? Are you using the newest version, overwriting all the files?