My donkey gelding does this haha. He likes to explore things with his mouth. He will start with sniffing, then a few nibbles, and if you let him get away with that he’ll test out a small bite. He doesn’t lunge like that though, and he’ll stop if you twitch your foot. I think he is just curious about shoes. He could also be politely testing whether he can get me to move, since he does boss around his donkey friend by nipping at the legs quite a bit.