Donnie seemed to have encountered some pony/creature suspicious and was about to put up a fight when the mysterious figure gave him a doll that was a human doll and he had no clue what it was, until they explained that it was an April from the 2012 Universe and with his expertise in dimensions, he immediately wanted to show every pony else the discovery and.. His crush who doesn’t love him back, April. But.. The moment he put hooves on the doll, he felt woozy and questioned it, but the figure dismissed it. The woozy feeling turned into a happy feeling, putting him in a state of hypnosis. And as he hypnotized from looking at the doll, his mature mind slipped away and soon enough, he had an infantile mindset. Once he started drooling and didn’t have the ability to form words, the figure poofs a diaper onto him in case he has accidents and no one wants to pick up puddles from little foals. But this gave the figure the opportunity to take him in and caretake, being a parental figure to them while they were babs.
The reason he got hypnotized and mentally regressed was because of the doll, it was enchanted, thus, any victim who looked into it, went into a state of hypnosis, and they acted differently depending on what was wanted. In this Stallion’s case, it was to be a bab and for the fact that he has the I.Q. of about 300, so he’s brainy. But of course, when he’s back to normal, he’ll be figuring out what happened and explain it like he always does.