We know absolutely nothing about Beast of Alaya, so it's back to the old fun speculation.

My guess is that whoever it is holds the title of Beast V, considering that it's counterpart (Beast of Gaia) is Beast IV. While the Seven Beasts and the Beasts of the World use different terms (Biisuto and Kaibutsu), it would be strange to have one Kaibutsu also be a Biisuto without the other also being one.

It could very well be Amaterasu, or maybe even Angra. All we know is that it resides on the human side of the counterforce.

well it cant be that two because two of them are beast candidates. like idk maybe a being who is responsible for humanity's survival wouldn't pick a fkin BEAST who by nature is the cancer of humanity to protect humanity you know