Board Thread:General Discussion/@comment-5899967-20150515220433

This might be a possible theory as to why Ruby has been forgotten. Maleficent and Lily are also girls that turn into "creatures", so perhaps it was to avoid a similarity, to make them more unique, to not be too repetitive.

The obvious differences are that Ruby only transforms at night, and her powers are out of control unlike Mal and Lily, though this was sorta resolved in season 2. 