Willow twigs? As in.. just twigs of willow.. don't laugh too hard, we don't get the things you lot get!
It is the leadrope chewing that gets me the most in case of injury!
Fact is that twigs and bark are part of horse's natural diet. Especially in early spring that's where the "juice" is.
I had a thief as others described....would take anything in his mouth, untie knots, steal buckets, farrier hats, take off with tools and when I was picking poop, would wait until I was off to get the last one and dump the full wheelbarrow then take off tail straight up, laughing. He did eventually quit....when he was 18....