A casual glance would say no, surely not. It looks like an ordinary dog with no particular resemblance to Papyrus - the proportions of its face are all different than his muzzle, it's smaller and fluffier than his canine form, white instead of reddish brown, and most of all, it's not skeletal. Papyrus sees only that first impression and rushes off after the dog, agitated to the point of missing what his nose - or Thancred - is saying.

But a harpy's eyes are more acute than an agitated turnskin's - or the average human witch's. Thancred might pick up on little signs of illusion magic, with the blurs for the mind to fill in, and the ways it's only as detailed as the caster could see.

At least, he might during the brief time that the dog's visible. But when Papyrus gives chase, the dog bounds away and ducks into a pile of crates some shop hasn't yet taken inside.

"Get out here! I saw that look," Papyrus yells at the crates, lifting one with the strength his bones don't hint at. "I know you know me. How long have you been here???"