[You forgot zombie zombies!]
In all seriousness, there are quite a few problems with this suggestion:
1) They really don't fit in the end
2) They could destroy an entire cow farm
3) They seem like pigman ripoffs
4) Do they attack the player? Do they have lots of health? More details, please!
Ultimately, while I somewhat like the concept of an epic end battle, zombie cows would not be the answer.