They can be best explained as Spirit the 2nd. They are both ghost-related, they attack by slowly flying at Momo, and are primarily white and red. Visually they differ by the yellow halo above Bakghost and the size.
Bakghosts also starts invisible and materialize when Momo gets close to them. Bakghost's size is a double-edged sword, as a bigger hitbox makes it easier to jump into it, but also to hit it.