As a child in the late 1970s, my family adopted a wild burro in California. My parents had her for over 30 years. I remember she had these freeze marks although you would have needed to shave it to read it. Burros are much shaggier than horses.
We also had horses. The burro was useful for training the horses to behave in a herd because she while she enjoyed being in the group she held herself outside of the horse’s hierarchy and would not be bullied
She was very adapt at wading into thorny blackberry bushes and eating only the ripest berries
I suspect the burros you encountered had been themselves harassed by dogs in the past.
Fucking colt snuck up on me from the back and bit me hard on the shoulder. When I jumped up to beat the shit out it the fucker did the old can't catch me nehhing business.
My quite short (~5') dressage champ younger sister had to punch way up when her horse was an asshole but I was very impressed to watch her discipline the horse.
I have hated horses all my life. My KLR650 is superior in every way.
That's a handy skill. Once you taste about-to-fall-off ripe ones, you can never go back.