I've not ever heard of a snake changing sexes. There are several fish species that can do this. And some snails are both sexes at the same time.I saw a news story maybe 20 years ago that A rattlesnake in the lab at Colorado State University changed sexes. The professor said that sometimes they do that in extreme situations. Has anybody else heard of this? I have also heard that sometimes animals can have offspring without the male's help.
Having offspring with no males is called parthenogenesis. I know of at least one lizard species that does this regularly.