If you want the variant of the mob to not change when leaving and comming back
on your world, what you can do is craft a Name Tag item, and then rename the
available list on the anvil, and then after you renamed the Name Tag, right
click on the mob of your choice.

make sure to refer to these list, it must not have a capital letter to work:

Rest of Mobs:
- male
- female
- mob


Cow, Pig, Chicken:
- male/female/mob
- cold male/female/mob
- warm male/female/mob