Quote Originally Posted by Sha View Post
I've just found a wild garter snake outside doing just that. It doesn't look wounded, but it is seemingly convulging in pain, twisting in circles around itself, lifting up its head and plopping it to the ground.
Have you gotten any closer look at it yet? Definitely no visible wounds? It is thrashing all the time, or only when you try to handle it?

If there's a herp vet in your area they could give a sedative for temporary relief and maybe figure out what's going on. But that's a lot to invest in a snake you don't know, that may be on its way out anyway. (That said, if they think it's really a dire case then at least they could quickly and humanely euthanize it.) A wildlife rescue/rehab center might also be able to help.

If you have someplace you could keep it (even a rubbermaid box with a secure lid) you could collect it up and keep it somewhere quiet and dim for a while, and see if it gets any better on its own. Left outside in that state it's probably an easy target for a hawk or other predator.