Atlantean Rat Snakes are a type of snake that can be found in the Enchanted Forest, one of which bit young Baelfire, only to be killed later on by Milah. The poison injected by this type of snake is fatal, unless cured by a certain, rare antidote.
Before the Curse
Baelfire makes a fatal mistake. ("Devil's Due")
One day, Baelfire goes out into the woods of his village while his parents are left alone arguing. There, he comes across a species of rat snake from Atlantis, which winds up biting him when he sticks out his hand. Baelfire cries for help, thus alerting Milah and Rumplestiltskin, who come to his rescue. Milah sees the snake that harmed her son, and promptly beats it to death before it can cause any further damage. When going to Fendrake the Healer about this predicament, Rumple and Milah are told that Baelfire's bite is fatal, but that there is a cure they can buy to save him. The couple do not have enough money for this cure, however, and so Milah sends Rumple back that evening to murder Fendrake and steal the cure. Fendrake catches Rumple though, but agrees to give the cure for free if he is willing to make a deal: Fendrake will get to have Rumple's second born child. Though Baelfire is saved, Milah is appalled by the deal Rumple has made. ("Devil's Due")