Sam would be wary, but that comes from experience with feral rats, which are best avoided unless absolutely necessary. Once he realized it was friendly, it would be a lot like an extra large mouse, sniffing at him to learn more about him and in general being a big fluffy buddy.
I think Sam would try and help it find its way back home because it probably misses its humans 🙂
And make sure Dean doesn’t freak out at the furry rat in their room XD
(That’s actually Dean’s boot at the edge of the gif XD)
Brothers Apart 