When a skeleton trap spawns, there will always will be one skeleton horse with a very strong health, jump height and speed stat. This usually only occurs when using the /summon EntityHorse command. The other skeleton horses have normal stats compared to naturally spawned horses.
Comments 2
Three of the spawned horses got randomized stats, but one of them failed to randomize its stats and got the unrealistic defaults. There's nothing wrong with wanting a powerful enemy, but it shouldn't be in this way.
This has been fixed now (they all appear to have 7 hearts now), but it was certainly a bug in its time.
Yes, they are tough enemies.