Animal Crates are a containment for Animal Buddies. Animal Crates were a signature feature in the Donkey Kong Country and Donkey Kong Land series and are first found in Jungle Hijinxs. Kongs are usually encouraged to release them (by jumping on the crate itself) to free the buddy and utilize them for the rest of the level. Some other Animal Buddies can be found in Animal Barrels.
The buddy the Animal Crates hold depends on the symbol on the crate. A swordfish would indicate Enguarde, a snake would be Rattly, etc. Animal Crates are usually placed in areas where the buddies would be useful, for example in Gusty Glade, Rattly is used to safely cross gaps. A more general example would be practically every Enguarde crate, who eases underwater exploration by being able to defeat foes underwater.
The crates only appeared in the Donkey Kong Country trilogy. These crates were also exclusive to the first Donkey Kong Land, the latter games did not due to hardware and configuration limitations. Oddly, in Donkey Kong 64, they work as the Animal Barrels. The Animal Buddies can't be released and when a Kong jumps into it, he will transform into an Animal Buddy.