There is no documented original recording, performer or release behind 'u mom horse' and the title appears to be a user-uploaded label from an online soundboard. Short nonsense pairings like this spread because they are easy to share as MP3s, remap to stream decks and drop into videos without any context needed. The 'your mom' setup gives it instant recognition, while the horse sound keeps it from sounding like a standard insult clip, which is why it survives as a small random humour sound rather than being tied to one show or game.