Bone Golem

This is a vicious shapeshifting skeleton. It attacks in three forms: First, what you see to the left; it'll elongate its head and punch at you, and it'll channel energy and spew a downward-looping flame. Next it'll become a vulture-like creature (top-right) that drops little orange skulls on your head while flailing its tail-attached limbs around. And last, a mound of skeletal parts that act as a blob; after it emerges from a mound of bones, it'll dash along ground, only slightly retreating when you whip at it. It'll continue dashing forward until defeated.