If Isaac has transformed into Guppy, Guppy's Hairball spawns Blue Flies when it deals damage.
If all the 9 extra lives have been consumed, Dead Cat is removed from Isaac's inventory. If Isaac only has three Guppy items prior to losing Dead Cat, then its disappearance will cause the Guppy transformation to be removed.
Transforming into Guppy as The Forgotten will give him the appearance of a cat's skeleton.
The guppy item tag determines if an item counts towards the transformation, except for Kid's Drawing which seems to be hard-coded to count as a Guppy item despite not having the tag.