Some mobs, in this case, the Panda, Horse, Iron Golem, camel, hoglin, zoglin, mule, turtle, skeleton horse, ravager, sniffer, polar bear, and zombie horse, upon spawn (or perhaps just upon spawn with an egg), get damaged by walls, and become it (which is some odd phrasing but i am unsure of which words to use).

https://1drv.ms/v/s!AqkhXlarV9-4dZj2qO5MCbN_DJE?e=ncZmlk