When creating a quest in 2.2.2 using tame mob you get the following error.

[22:47:36] [Server thread/ERROR]: [Quests] Horse inside mobs-to-tame: inside Stage 1 of Quest New Horses is not a valid tameable mob!

First, try using the latest version of Quests (2.3.0).

This continues to be a problem in 2.3.0. No errors in console of any kind until /questsadmin relog and the same error. The error shows up in the yml file but will not load as a valid quest when doing /quests list (seems obvious why i know).

Here is the quest from quests.yml for your reference:

custom1:
name: New Horses
npc-giver-id: 34
redo-delay: 604800000
ask-message: I always need horses for my stable. Can you tame me 5?
finish-message: Thank you. Come back next week. Here are some apples, hay and 250 ozonebucks.
stages:
ordered:
'1':
mobs-to-tame:
- Horse
mob-tame-amounts:
- 5
rewards:
items:
- name-GOLDEN_APPLE:amount-2:data-1
- name-HAY_BLOCK:amount-3
money: 250
quest-points: 1