Sail Back Boi vs Big Millipede Boi

neither

because they weren't alive during the same time

Ik, but if they met. Who would win

depends

if it was during the permian, the dimetrodon would win because it'd actually be able to breathe.

If it was during the carboniferous, the arthropleura would win because the dimetrodon would die from oxygen poisoning

Hmm it depends which of these animals would win?