Unable to attach a Lead to Undead Horses

I am unable to attach a lead to undead horses. I haven't actually come across any regular horses yet so I am unsure if this effects them.

There really is no step by step, I am able to attach leads to chickens. I only checked them as they were the closest. When I attach a lead to an undead horse, it goes into the horse's inventory instead. I've tried shift clicking and the same thing happens.