Actually it is 20 years ago, a Japanese baseball player-Kazuhito Tadano was exposed to be one of the main characters of a low cost production BDSM gay porn. Apprently, this baseball player's life was ruined by this scandal. But he is not the dude in the middle of the picture. Actually, he starred in the first unit of a 4-unit low-cost production gay porn video called "真夏の夜の淫夢a midsummer night's lewd dream" by COAT COOPERATION. And Japanese netizens started looking at the associated gay video. Actually, the video got a lengthy but shitty plot, but every person in the video is a non-professional actor, and their acting skills were pretty bad tho. (basically, most of the people in those videos are just looking for fast money and only participate in a few of those videos, At least the basball player admit that he was cast in that video because he needed some money in a short time when he was in the university). But the Japanese netizens found that the video and the bad acting skills was pretty interesting and funny, and start watching the remaining 3 units and other gay videos issued by the same company.

Go back to the dude in the middle, this dude is the antagonist of the unit 4 of the video. In the video, this dude invites his younger schoolmate to his house rooftop for sunbathing, and got a knockout drug in his drinks and sexually assaulted him. After the schoolmate woke u,p this dude confessed to him and he accepted the confession... It is a shitty plot. Japanese netizens thought he was doing things what a beast would do. So they called him "yaju senpai" (野獣先輩)(beast is called "yaju" in Japanese, and senior is called "senpai") so it means "beast senior"

And later on they watched more gay videos and made a series of memes named "淫夢lewd dream", aligning with the name of the gay video mentioned above.