You should load the animal object, which you want to update. createForm() will use the loaded object for filling up the field in your form.
Assuming you are using annotations to define your routes:
/**
* @Route("/animal/{animal}")
* @Method("PUT")
*/
public function updateAnimalAction(Request $request, Animal $animal) {
$form = $this->createForm(AnimalType(), $animal, array(
'method' => 'PUT', // You have to specify the method, if you are using PUT
// method otherwise handleRequest() can't
// process your request.
));
$form->handleRequest($request);
if ($form->isValid()) {
...
}
...
}
I think its always a good idea to learn from the code generated by Symfony and doctrine console commands (doctrine:generate:crud). You can learn the idea and the way you should handle this type of requests.