Does buwch mean specifically female cattle or can it refer to oxen/bulls as well? How about females from other species (maybe buwch forfil or buwch eliffant)?
Buwch is the female i.e. "cow". A "bull" is tarw and "ox" is ych, bustach or eidion.
For "female elephant" you could use eliffantes, but I think eliffant benywaidd would be more common. A "female whale" is morfil benywaidd. If you just want to refer to a "cow" in this context then it's benyw i.e. "a female".
"Hwyaden" is the singulative of "hwyaid". The plural of "buwch" is "buchod".