The zoo near me a few decades ago had an accident where a lot of their reindeer got pregnant before the male could get neutered. They kept all the female offspring but they couldn’t risk having two males in the same enclosure and no nearby zoos needed male reindeer or also couldn’t accommodate them so they had to put them down and ended up feeding them to the carnivores in the zoo.
My zoo buries them on the premises. I know of a camel, a moose and a few other things got buried there. They have a large plot of land that is used to dispose of organic waste like branches and trees, old compost, etc until it’s full and needs to be tricked out to the landfill. They just bury the animals under the ground there.