What is the correct method for breeding Donkeys??

Hello!! I was an avid player when when I was a kid and I have returned many times to the game through the years. I managed to login to one of my old accounts that wasnt deleted. And seems like I was breeding Donkeys (or trying to) but I am unsure of how to go about it, since they loose GP with every new gen.

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What are you supposed to do with them?? Xd