An animal hood is a hood designed to give the wearer the features of an animal, often a dog or a pig. Animal hoods are often used as part of an animal transformation fantasy, both to add realism and to act as a restraint, forcing the person to stay in role. 
Animal hooding can be frustrating for the transformed person. The wearer may be tempted to remove the mask. In cases where realism is enforced for the wearer, complex locking systems, collars often of made of steel, rotary locks and independently locking zippers that can conceal and locked from access should be used. Then every so often, attempts at escape should be allowed even if these attempts are without the Handler's knowledge to re-enforce notion they are what they look like and change it is beyond their ability to control. Mentally this can also help ease the frustration.
Mind Control Edit
Release should be restricted as much as possible. Illness and other necessities, family and so forth that wouldn't understand MIGHT be reasons for release. But your job as a Handler is to change the mind of the animal, to make them believe they are what they see. This is a form of mind control and must be entered into it mutually. It should be understood by the animal that your intention is to change the nature of what she is and make it unbearable to be anything but the animal she has been changed into. and that personal peace can be found only as she is.