Sly notices the cat, smaller than he is, but recognizable enough as a cat, and cats can be dangerous... and he notices the other before they do him. He does not seem to know that the TY-eared creature is not a real cat. If real cats are a thing that can be determined from not real cats.
Sly puffs up, standing on all fours and on tippy toes, tail sticking straight up into the air and mouth open wide. He remains stanced like this and jerkily backs into a corner making hissing clicks before the cat does something strange... speech... The monster calms immediately. Animals, generally, did not talk. He suppresses his defensive posture and takes a couple seconds to reply, confident more in the availability of verbal communication. Sheepish at the immediate resort to discomfort towards this stranger.
"A wrong turn... yes... yes... I am here somehow but I cannot leave. A room..." He twitches the tip of his tail ever so slightly and gives the baseboard a hard shove with a front claw.
"Depends on where we are... No end to Earth, if this is Earth... still... but Underbed has an end. Beyond the Wilderlands... I think..."