Animals typically only wear clothing to amuse their human masters -- and yes I mean "masters" not "companions," "guardians" or "parents." For to clothe an animal is to bend the creature to our anthropomorphic will. Trust me, your cat does not want to go trick-or-treating and dogs will not play poker.
Still, occasionally science finds a reason to dress an animal in something resembling human clothing. Or sure, it's generally a matter of containing the creature's senses or functions, but let's not kid ourselves about the results.
So here are eight examples of animals wearing "clothes" in scientific experiments. I'm skipping the various examples of animal-tracking backpacks, because you didn't come here to read about that. You want frog pants, and I intend to give them to you.