The bug
Zombie horses don't sink in water.
Linked issues
is duplicated by
Attachments
Comments
@unknown, source?
Snoggie, the page for the snapshot said ALL undead mobs. Zombie Horses not sinking is, therefore, a BUG.

Affects 18w20a

Affects 18w20b

Affects 18w20c
Confirmed in 18w22b.
This is intended.