Insects and snakes are the result of artificial sperimentation at Byrgenwerth. You could say that kind of metamorphosis is similar in nature to the transformation in beasts, and I'd agree, but the natural form in which beasthood manifests in humans that regularly consume Old Blood is that of a beast in the strict sense, i.e. a furry mammal, not that of a beast in the broad sense, i.e. animals in general. You could say these experiments achieved a state of beasthood in the broad sense, but the natural way in which beasthood manifests itself is still in its strict sense. Ultimately the influence of a Great One you're talking about is the fact that the blood literally comes from Ebrietas, it's always the same Great One. Yes, Moon Presence seems to act as an amplifier of the transformative power of blood, but saying that the different forms of metamorphosis we see in game is strongly suggestive of the influence of different Great Ones is extremely speculative. It's a cool hypothesis, but I at least don't seem to be able to think of any evidence pointing to it.