While you ponder what to do with the strange costume, your dog walks into the room, tail wagging happily at the sight of you. He comes over expecting you to pet him, but as soon as he nears that horse costume he starts growling, the hairs on his back bristling.
"What? What is it?"
He ignores you, still growling, pacing his way around the costume as though it were a wild bear preparing to rip your throat out and he was the only thing standing in its way. You try to shoo him, but he refuses to leave and just keeps staring at the mass of latex on your table. Then, like a trained attack dog, he leaps past you and onto the table, clutching the costume with his teeth and dragging it to the floor.
You run around the table to check on him, but he's badly tangled by all the rubber. You start moving it around and you can see his shape under it and hear his muffled whines, but you just can't seem to get to him. Then you realize: if he can't get out, he could suffocate!
Worried for your little pal, you rush into the kitchen and grab a sharp knife. If you can't dig him out then you'll just have to cut your way through. Running back to the room where he lies on the floor, you prepare to slice through the costume... then stop yourself when you see it. The mound that was your dog inside of it has started to grow, filling out the suit. It doesn't take long for him to reach the right size for it to be a perfect fit, his muzzle still gnashing and biting within the horse costume's. Soon, his sharp teeth are made duller, the latex adhering to him as though it were becoming his skin. The legs fill out, soon gaining a muscular shape like those of... well, of a horse, and the "arms" do the same, becoming a pair of horse legs themselves. The panicked yelping you'd heard shifts into a thick, terrified whinny while he grows and reforms with the costume to form a far more appropriately-sized stallion.
You just stand there, stunned, as he calms down and brings himself to his hooves. Your once adorable little puppy is now a massive brown rubber horse, with its long tongue hanging out the side of its mouth and its tail wagging as best as it can.