Hm. "Bullock-look" could be oxy-something ("ox see"), but I don't see any that start with that. Or it could just be "cow," but Coumarin is too short for the rest of it unless "mar" can cover "look-looker-on" somehow. (I considered trying to fit "cow mate" into Coumatetralyl, but bullocks are generally castrated so they don't mate with anything.)
Oxy is a good idea, i got to
oxybutynin, which treats overactive bladders which would be in line with the dialogue (something like 'i didnt see them dead but there was less filth the day after') - which to me sounds pretty perfect, but what is Butyn?? but-in a conversation? no clue how that links. maybe Button? again... idk
oxyimperatorin, which is another coumarin and thus would definetely kill a rat (lol) but again 'imperator'??? Also, it seems pretty niche so i dont think companies would make rat poison out of it (trying to think of whoever put the quote in the game and where they got it lol)
i definetely think if it is one of those, its oxybutynin. very common medication and lines up with the clue.
still no idea about 'butyn' though
perhaps 'looker' and 'on' are two seperate parts, not just 'onlooker' as in spectator.