The bachelor stallions kill ANY foal so that the mare will come back into heat sooner. The sex of the foal doesnt matter. If a new stallion takes over a herd, the first thing he does is kill all the new foals.
Lol, I think you've got feral horses confused with African Lions. While stallions have been observed to kill foals -Including one particularly notorious case in this herd- it's ludicrous to say that bachelors kill foals just to bring their mothers back into heat. Mares naturally go back into heat within one to two weeks after foaling! It makes no evolutionary sense for stallions to kill foals for that specific purpose when mares are receptive again so quickly after birth.
Hell, this herd in particular has numerous accounts of foals
definitively known for being by one stallion being raised by another! One rival pair ended up raising each others firstborn sons! And hilariously enough, one of said sons didn't even leave his natal band until he was
five years old. His "step-father" never kicked him out!
And in another herd, I watched in real time a pair of bachelor stallions
adopt an orphaned filly! When she was two months old, her mother was struck and killed by lightning. The rest of her herd survived, but ended scattered across a wide area and never joined up back together. The filly mustn't have gone far, because she was observed standing by her mothers side for several
days afterwards. She would've died if the bachelors hadn't stumbled across the incredibly sad situation by chance. They led her away from her mother to a nearby waterhole and then proceeded to stick close to her for
over a year. They looked after the filly- Played with her, kept watch as she slept, led her to food and defended her from other horses and
humans alike. It was really quite moving, they took such tender care of "their" filly that one would've thought that they were her natural fathers.