The Immoral Beast is a boss in the video-game, Catherine. It takes the form of a grotesque mishmash of legs conjoined together with eyes and a nose on its back, and a mouth on its front with an elongated and exaggerated tongue.
Introduction
A red and gloomy area, Vincent enters the second boss battle. A roar shakes the blocks as Vincent looks down to find a tongue lashing out at him, attached to the Immoral Beast. This begins the final nightmare of the third night.
Meaning
The mishmash of an overtly sexualized creature with a female face (that has Catherine's eyes) make it clear this boss represents the affair that Vincent commits the night before with Catherine. While specific sexual acts can be speculated on based on the physical appearance of the boss and Catherine's admission the next morning that she has never done something 'like that' before, it is left up to personal interpretation as to what it really means.