Java Edition:Skeleton Horse with Poison or Regeneration
Skeleton Horses with Poison or Regeneration were obtainable before they were considered undead mobs. The Poison or Regeneration effects will eventually run out, making this a temporary discontinued feature.
Obtaining
In 15w38a (1.9 snapshot), the Skeleton Horse was made obtainable in survival from skeleton trap horses that sometimes spawn after a lightning strike. In 15w38a, Skeleton Horses were not considered undead mobs which allowed them to get affected by poison and regeneration. In the next snapshot, 15w38b, they were made undead mobs, so poison and regeneration can no longer be applied to them.
Variants
Poison 1 can have a duration of up to 1:30 and poison 2 can have a duration of up to 0:21.6 using splash potions. However, if a skeleton horse is downgraded to between 15w31a and 15w32c then poison 1 with a duration of up to 2:00 and poison 2 with a duration of up to 0:22.5 can be obtained using splash potions.
Regeneration 1 can have a duration of up to 1:30 and regeneration 2 can have a duration of up to 0:22.5 using splash potions. However, if a skeleton horse is downgraded to between 15w31a and 15w32c then regeneration 1 with a duration of up to 2:00 can be obtained.
In later versions, the duration of the potion effects can be decreased by loading the skeleton horse but the duration cannot be increased.