I'm thinking maybe it was a stuffed Porthos on display with a statute of Archer and Scotty was tinkering with the transporter and tried to beam the stuffed beagle and it didn't work.
That one, I could live with.
I'm sorry, but I don't understand why some of you can't "live with" it being a real beagle? Scotty obviously wasn't doing it to be cruel; he was probably very confident that it would work.
Scotty also used some kind of animal (it was a dog with a horn or something) as a test subject in the transporter in "The Enemy Within". That didn't work out too well, either.