IK the mark on his back looks like a turtle shell, but it's supposed to be a radioactive-like symbol Lol I also thought of the poison dart frog idea and the swirls- so why not give some swirly-like thorn-like markings? maybe even spikey/lightning-bolty markings? I also gave him one darker paw, as that could mean "caught you red-pawed" as a small detail...
