Java Edition:Stacked Horses

Warning: April Fools' 

This feature requires April Fools' versions and may require the use of file modifications to load. See the tutorial for information on loading these versions.

In the April Fools' snapshot 15w14a, it was possible to get a stack of horses. This can later be done in April Fools' snapshot 22w13oneBlockAtATime, though much harder to replicate.

Obtaining

15w14a Horse Stacking

In 15w14a, horses could be stacked on top of the player by right clicking them.^[1] They can be removed from riding the player by logging out, or submerging the stack into water. While both tamed and untamed horses could be added to the stack, untamed horses will randomly dismount their mounts, making them a temporary discontinued feature if the stack consists of untamed horses.

22w13oneBlockAtATime Mob Wrestling

Warning: April Fools' 

This feature requires April Fools' versions which may require file renaming or file deletion to load. See the Loading April Fools' Versions tutorial for more information.

In the April Fools' version 22w13oneBlockAtATime, the player can pick up nearly any entity to create mob stacks in any combination. This powerful method also allows for stacked horses to be obtained.

References