Java Edition:Baby Skeleton Horse
Baby skeleton horses were first added to the game in 13w16a, but never made encounterable in survival mode. They cannot be bred from adult variants nor they can spawn in skeleton traps. However, a handful of April Fools' versions add methods to obtain this rare mob.
Obtaining
20w14∞ Dimension
One of the random mobs that the dimension can spawn is baby skeleton horse. Known place place where baby skeleton horses can spawn is "_generated:773938501" biome at X 1357 Z -1126 in the "q3" dimension. Once found they must be transported to another dimension that will stay after updating later. As other horses, baby skeleton horses eventually become fully grown. Thus they need to be put in unloaded chunks to keep them as a baby for longer.
Spawn Egg
The skeleton horse spawn egg may be obtained in 22w13oneBlockAtATime and 23w13a_or_b, which may be used to spawn a baby skeleton horse by simply right-clicking on an adult skeleton horse.