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I'm sure everyone assumes this one but: Male Kangaskhans are Cubones. There is something in the shape of the male that kills the mother in childbirth, and leaves the baby Cubone with an under developed skull. They need to take their mothers otherwise their soft and squishy one would cave in on the first hit. Eventually it fuses to their head and face and becomes part of them (Hence Marrowak and the "nobody has ever seen its face thing)
I'm working on a Pokemonster Hunter table top game for some friends, so we've been trying to add some realistic touches. Here's one I have for Machoke.
Also, the belts on Machokes are an external organ, calloused and hard on the outside, but on the inside have their testosterone and adrenaline glands. Also, they have a secondary pectoral muscle that goes from upper chest, down to that belt, to give more force on downward punches.