There are ten species of cat-eyed snake. I believe this one to be the northern cat-eyed snake. You can see where they get their name with that vertical pupil. Cat-eyed snakes are venomous, but the toxicity level is very low and only dangerous to very small prey. I came across this guy in Corcovado National Park in Costa Rica but they range as far north as southern Texas.
Nikon D800 with Nikkor 200-400mm lens (at 400mm) ISO 800, f/6.3 at 1/60th of a second, two Nikon SB-900 flash units