Another vote for Cottonmouth. Why take the chance? If I don’t KNOW it’s not poisonous it’s dead, period!!!! A young Rattlesnake killed our Yorkie years ago. She had 18 hours of suffering. A business associate lost their Pomeranian last year to a Rattler. We saw a Corn Snake in our gardens yesterday, picked it up, played with it a while and turned it loose in a more remote area of the yard. Have you ever watched that show where it shows people in the hospital that have gotten bitten by poisonous snakes? If you have, then you know why I say kill it if there is any doubt. It’s not worth losing a beloved pet or God forbid, a family member.
A few years ago our German Shephard killed a large cottenmouth in our backyard - it had wondered into our mulch filled playground set where our kids were playing. We couldn't tell if he was bitten (the dog, not the snake - the snake was bitten in half), so we rushed him (the dog, it was clearly too late for the snake) to the 24hr Vet. That's when we got a quick and valuable lesson about dogs and snakes.
It's not like people where the antivenin works everytime. It doesn't always work and I understand it can have deadly side-effects as well - plus it's ridiculously expensive. Several hundred a vial and a 120lb. dog could take as many as three, perhaps even four vials.
We stayed at the vet for a couple hours, but it luckily it turned out the dog wasn't bitten.