The user can transform into animals, as well as use the abilities, traits and appearance/physiology of animals by rearranging their own DNA structure. Users may be able to transform into alien animals and/or animals they have never encountered, such as Dinosaurs. Some may have this ability from an empty genetic code, allowing them to accept any form from which they have a DNA sample, others may be able to alter their form mentally and change just by seeing and mimicking animal features.
Kumiko Ishihara (Buffy the Vampire Slayer Season Eight)
Aximili-Esgarrouth-Isthill (Animorphs)
Jake Berenson (Animorphs)
Rachel Berenson (Animorphs)
Tobias Fangor (Animorphs)
Tom Berenson (Animorphs, later in series)
Marco (Animorphs)
Cassie (Animorphs)
Visser Three (Animorphs)
Andalite (Animorphs)
All Yeerks (Animorphs, later in series)
Zoan Class Devil Fruit users (One Piece)
Druid Class (Diablo II: Lord Of Destruction / World of Warcraft)
Druid Class (Warcraft)
Monkey Talisman (Jackie Chan Adventures); it can also reverse the effects of it's own abilities
Bayonetta (Bayonetta)
Jeanne (Bayonetta)
Werewolf (Lost Tapes)
Skinwalker (Lost Tapes)
Simone and Lena (Scooby Doo on Zombie Island); Morph into Cat-People at a full moon.
Angel (Maximum Ride) (Has only been mentioned once)
Pain & Panic (Hercules)
Rin Kaenbyou (Touhou Project); a cat youkai, who actively turns into her cat form (most youkai in Touhou are humanoid)
Remilia Scarlet and Flandre Scarlet (Touhou Project); as vampires able to turn into bats. Remilia is able to turn into an entire swarm of bats, and is able to recover from just one.
Keine Kamishirasawa (Touhou Project); is able to transform in a were-hakutaku during full moon.
Dracula (Dracula)
Lisana (Fairy tail)
Chaos (Sonic Adventure); has animal forms when in possession of 4 and 6 of the Chaos Emeralds, Perfect Chaos - which uses the energy of all 7 of the Emeralds - is a huge lizard.
Nidalee (League of Legends); Can turn into a large feline - the exact species of which depends on what skin is in use at the time.
Elise (League of Legends); can freely transform between Human and Spider forms.
Shapeshifters and shapeshifter-chameleons (Shadow Falls)
Perry Gomez
Kevin
Roberto Esparza
Rebecca Antoinette (UN Comics)
Cara Cruz (Wansapanataym Season 1 Episode 8: Kokak)
Gallery
Titania Caban (Marvel) emerges as her animalistic manifestation, Catiana.
From World of Warcraft - the Bear Form is the first shapeshift ability learned by a druid.
Beast Boy (DC)
Beast Boy (DC)
Animal Man (DC Comics)
Snowbird (Marvel)
Jake Berenson (Animorphs)
Rachel Berenson (Animorphs)
Tobias Fangor (Animorphs)
Tom Berenson (Animorphs)
Marco (Animorphs)
Pain & Panic (Hercules: The animated series)
Yoruichi Shihoin (Bleach)
Zoan Class Devil Fruit (One Piece)
Werewolf (Lost Tapes)
Catiana (Marvel) can take a form of animals after tasting their blood
Lucy (Misfits) takes the form of a mouse.
Lisanna (Fairy Tail) with Take Over Magic can change to any animals