I mean as well didn't she mentioned she needed to steal a beast's crown? Cause she isn't trying to become the "natural" beast type but a completely new type of beast(Which would mean that Amaterasu is an existence that could be Beast V, but she isn't actually it yet, basically it's that you are potentially qualified for the position, but until you take up that position, you are not an Evil of Man until you or someone takes that position), that means she kind of needs to collect all the pieces to become a beast of her own creation, which the beast crown(a Beast's horns) being one of those things she needs and I doubt she would be able to steal a beast's crown by herself(Which would require the beast fully manifesting into this world, take note that it takes time for a Beast to fully manifest/mature, take for example Kama's stage where her beast's crown had an immature and matured state.).
It's fairly likely that there is a Beast lurking in this lostbelt cause it's one of few entities that would be considered a threat to the Alien God(Because despite the beasts being of the same in being threats of humanity, they are all self-dependent and do not have any ability to coordinate/cooperate with one another), cause there was no concern shown to Daybit's lostbelt, despite I'm sure the crypters would definitely be aware of the entity known as Type-Mercury, or at least be made privy to it in some manner, and it bears mentioning that Primate Murder and Type-Mercury despite being of similar tiers of threats, Type-Mercury is not a beast. Basically while Primate Murder is on the same tier as Type-Mercury, it is not Gaia's Ultimate One, that would be more closer to Archetype Earth than anything else. Just that their abilities to kill humanity(Or well I guess you could say, civilization if you want to use Altera as an example of what it likely means) are comparable to one another.