Animal roleplay

Animal roleplay or pet play is a form of sexual roleplay, where at least one person behaves and acts like an animal. It is common in the context of BDSM, where the submissive person usually acts like the animal, and the dominant person plays its master. The animals that are imitated usually are domesticated animals. 
The people who imitate the animals are either completely naked, or they wear special clothing, for example clothing that imitates a harness for ponyplay, or a leash for dogplay/catplay.
Other forms also exist, but are less common. Note also, that this is a form of roleplay, between adults who agreed. It is not about sexual relations with animals (which is called zoophilia, and forbidden in most countries).