I think the Robo-crocs might only after a certain time into the storyline, not sure. The turtles can be seen at beaches I think, but there is actually a pretty funny side mission where you kill some anyways.
The snakes were the hardest for me, I got lucky and randomly saw one as I was running up a mountain once.
hunting mutated turtles is one of the sidequests you get from one of the southern garrisons, and during it you will come across a robo croc.......and boxes of pizza. Be sure to read the description for the mutated turtles after you kill one, it's pretty funny.
As for neon snakes, either in the water along the coast, or you can find them along the grassy hilly area. If you don't see one, and one is near, he'll jump up on your arm and say hi.