As the title says. I'm trying to find out how to make it so that Cows after being given 3 of a custom item become a custom mob I made. Kind of like Twilight Forest's transformation powder that changes normal animals into the twilight animals. 
Also if there is a way to have it so I do not edit the base files. 
Only way's I have thought up is somehow with Ocelot's being given fish and made into a cat. Or make it so the custom item is set to change the mob. 
(Sorry if there is some obvious answer I just cannot see it) 
I need help changing Cows but not the base file so that with 3 of a custom item it becomes a custom mob.