yeah, it's actually a dog giving head to a guy
If you read the book it explains a lot. Here's IMDb's explanation:
Like the woman in room 237, this mysterious scene is explained in the
novel, but not in the film. At one point in the novel, Jack is dancing
with a woman at a
masque ball
during the 1920s, and he notices a young man wearing a dog mask and
behaving like a dog for the amusement of a tall, bald man. This bald man
is the man in the tuxedo later seen by Wendy. The woman explains to
Jack that his name is Horace Derwent, a former owner of the hotel, and
an eccentric
Howard Hughes
type figure who poured over three million into restoring it after WWII.
The young man acting like a dog is Roger, a former lover of the
bisexual Derwent, with whom he is still in love. According to the woman,
Derwent told Roger that "
if he came to the masked ball as a doggy, a cute little doggy, he might reconsider;"
that is, he might have sex with Roger. Although no actual sex scene
between Roger and Derwent is described in the book, such a scene does
seem to take place in Kubrick's film, albeit obliquely.
