When you mount onto a zombie horse the message action.hint.exit.ZombieHorse shows up.
MCPE-17212
When riding a zombie horse you get the message action.hint.exit.ZombieHorse
Resolved
Fixed
0
(Unassigned)
Unconfirmed
Tablet - Android - Samsung Galaxy Tab S
0.15.90.7
0.15.90.8
Retrieved
Assuming this has been fixed since you can't ride zombie horses again.