For tagging on this site, this is clearly "monster girl" because the character's not an animal. In a "deeper" sense, it's irrelevant, the lower body's gonna be too animallike for some monster girl fans and the upper body's too too monsterlike or humanlike for some bestiality fans. Boxes like that will always have fuzzy edges. Unsurprising when furries and beasty boys are involved, these people love some fuzz in their porn.
Are you asking from a general perspective? No, because the idea behind the term and its stigmatization is that animals lack the abilities to understand the complexities of human relationships and effectively communicate their needs. It's impossible for an animal to provide informed consent to a human. The exact configuration of their genitals isn't what makes it bestiality or not, it's whether or not they can interact with a human on a level playing field. Unless that centaur lacks human-like cognition, then she does have the ability to interact with a human as a peer, which makes it not bestiality.
I think that gets the order wrong. People don't have strong feelings bordering on disgust about bestiality because of abstract modern theories about consent that nobody believed for most of human history, they have strong feelings about bestiality and that motivates them to reason themselves into why it's wrong. Not saying your reasoning is wrong, just that it's one among many.
Maybe biology plays a part in those feelings, but cultural variation suggests it's more about culture. Prolly it's no coincidence that the civilisations that were most influential in constructing modern law and ethics had a few centuries of religionist regimes that put people to death for bestiality. So in a way you're putting a progressive cart before a conservative horse.
So it's too subjective as a benchmark for deciding what's bestiality or monster girl fetish.
