base fabric 1.20.1 instance
fabric api, more mob variants (1.2.2)

while mob textures are unaffected it appears the game is unable to locate default textures for chickens, cows, pigs, sheep, wolves, and zombies, leading to missing textures within mob spawners, and the log spits out errors regarding them:

relevant log details.txt
latest.log

cat spawners however don't seem to have this issue

This'll be fixed in the upcoming 1.3.0 update.