Holly was originally a semi-voluntary shapeshifter: she became a cat and a chihuahua onscreen, temporarily at will, and then every night regularly! Like mermaids, she was susceptible to the influence of the full moon, and gained superpowers unrelated to her shapeshifting.
Davilin controlled the element of darkness, and could steal powers from those who had unlocked them. He also possessed the ability to control minds, as well as possess others.
Cleo was originally a mermaid, and still apparently considered herself one after gaining this power. She used it by accident several times.
Talie Kresh and her sister Marlowe were born humans. Discovering a half-white, half-black rock, the two fought over it, ending up so each touched one side. At that moment they transformed into shapeshifters--at the same time, the two became embodiments of good and evil. When Talie Kresh took on the form of Aly, she also gained Aly's broken leg. She was also a hypnotist capable of wiping memories.
For unknown reasons, when Marlowe shapeshifted into a mermaid and got wet, she lost her other shapeshifting powers. Since her shapeshifting teacher had warned her something bad would happen, it can be assumed this is a common problem for shapeshifters.
Main article: Sirens
Some shapeshifters are also designated as Sirens, often because they also have mermaid tails or hypnotic singing voices.