Sorry for the previous outburst. But I’m still confused who keeps doing this. I already crossed off Caramel, Noteworthy, Cherry Fizzy, Big Macintosh, Shinning Armor, Pokey Pierce, the mailcolt, Braeburn here, Time Turner/Doctor Whooves and Carrot Cake. There’s only 5 other males I know left. Soarin, Roid Rage, Thunderlane, Fancy Pants & Blueblood.
I got another theory:
The stallions that are captured by the changelings are filled by eggs until there’s no room and turned into slaves doing any kind of chore, from forced work to various kinds of entertainment.
The stallions must subdue to they new masters if they don’t want the eggs to hatch inside of them. What they don’t know is that these eggs they have stored aren’t fertile, so they are lickely to stay bloated until someone frees them.
The jelly that coates the eggs has nutritious and aphrodisiac propieties that is absorved by the stallion body during the time he’s being enslaved, feeding and hydrating him from the inside. The changeling ichor is fueled by “love”, so if the stallion is “loved”, the jelly will regenerate and the host will never need to find any kind of nourishment from the outside. The ooze’s aphrodisiac propieties are able to eventually change the host body permanently.
A stallion newly enslaved will feel uncomfortable or even sore by the fact that he’s bloated with eggs. After some weeks his body and mind will adapt to the eggs and he will be able to move freely even with his bloated body. Finally, after some months, the slave will be unable to think about living without the eggs, even there are odds that he will try to get filled even more, pushing himself to the limit in order to get more eggs as reward.
At that point there’s no turning back to their original selves, so freeding them from the changelings will no revert them back and they will seek for another “master” to serve. That new “master” doesn’t need to be a changeling so there’s a brand new slave market where the changelings sell fully brainwashed slaves to anypony who could afford it. No questions allowed.