it is wrong to call a waddling_head character of feral or anthro. thinking about it, I think it will be better to classify it as a form. and we could also create the tags (waddling_head_penetrated) and (waddling_head_penetrating).
even though this tag still doesn't qualify as a form, I can't think of a reason why it couldn't make it into the x_penetrating_x classification since human is also a unique shape tag that does.
feral = animal with the body of an animal
anthro = animal with a human body
humanoid = creature that looks human with minimal or no animalistic features
waddling_head = a character with a round body that resembles a head.
but it's like I said, even if it's considered a specie, I still see no reason why it wouldn't fit into the x_penetration_x category.
Updated