Mares give a quick nip to the hamstring of roughly nursing foals to remind them to be gentle. However, as shown in the second image, mares are more often gentle and attentive with their babies. This is the same mare and foal, in case that’s not obvious.
A quick nip to the hamstring reminds this foal to be gentle when nursing. A good mother is also one who disciplines her foal for rough or disrespectful behavior.A good mother gently nuzzling her nursing foal.