Describe the bug
Make sure to include:

1. What did you expect to happen? Horse to tame and be rideable
2. What actually happened instead (i.e. what was the bug): Nothin

To Reproduce
Attention to detail is important!

1. Familiarize a horse
2. Get on it's back
3. .... do not profit

Issue resolved in debug by adding a super.initEntityAI(); to Horse. presumably the same for other mountables.

Version 10.139

Actually looks like the task that it needs is EntityAIRunAroundLikeCrazy (because of course it is). Only needed for Donkey, Horse and Mule... Camels get the task added through their llama heritage