They're modern knock-offs using a vintage 80's newborn pony mould. There's no particular reason there's so many colours other than, one would imagine, it's an easy way to give variety to a product rather than the more expensive option of varied paint/poses. They're popular for customs since they use an original pose, and that's part of what makes them popular for fakie collectors too I imagine. Most original fakie moulds aren't as cute as the Hasbro ones.