Where can I find the "mutant donkey woman" horse?
I watched this video where the players found a 'Mutant Donkey Woman':
Where can this person/horse be found?
After patch 1.02, the mutant donkey lady can no longer be found. Before that, she could be found northwest of Chuparosa.
I don't believe there is any way to deliberately cause this to happen. It is most likely caused by the game confusing the models for the horse with the models for the man or woman, and it doesn't seem to happen to a specific NPC. I think the only thing you can do is keep playing until it happens. (If it happens).