When the mounted Equis takes damage there is a chance that the taming progress will be reset.
Getting the Equis trapped in my quetzal's hit box gradually lowered the taming progress - this did not happen when it got stuck in wild creatures.
Once an Equus is spooked it tends to run forever and not allow it's self to be tamed.
You can ko the Equus for another passive tame attempt when it wakes up.
Equus seems to spook easily at night and spawn mostly during the day.
Feed it then mount. You won't be able to control it until the tame is done, dismounting appears to lower taming effectiveness so make sure you have a good supply of carrots (preferred food) on you. When it starts to buck you off you have to hit enter to feed it again. If you pay attention to the bottom left corner of your screen there will be a tool tip that will pop up when you can feed it abs just before it starts bucking.
These guys are easy to tame low level but a 150! The tame percentage drops as the horse runs do good luck ever taming a high level one. Unless! You knock it out build a box around it wait for it to wake up and the tame to your hearts content. Vote up to help others!