Responses
It's honey water, Seunghyun ignores it at first but ends up taking it to his room and drinking it. It's good for cold/hangover, the author uses the same motif in PB.
Pet milk is just a substitute for regular milk, Yuta is not going to hurt Tora, there are other characters that will be villains in this story.

He sees the cup at two places. Once when entering home at the countertop and next near the bedside table. What's in the cup!?
Also what was that pet milk thingy did yuta do something to the dog!? He can't be that cruel right?