The gemara states that being bitten by an animal is a תולדה of שן. However, Tosfos points out that this would not include a snake bite which would be a tolada of keren. This relates to the sugya in Arachin 15a we learned where the animals discuss with the snake how it does not get הנאה from its' bites.
However, the Rashba says that a snake bite can still be a tolada of שן. The reason for this is that the snake still has הנאה even though it does not digest. Thus this explanation still works with the gemara in Arachin since there it discussed eating and not biting.
Just to round out all the possibilites, Tosfos on 16a says that a snake bite is a תולדה of רגל since this is a normal act of a snake and Tosfos therefore says that if it is רגל this would mean that you would be פטור in reshus harabim if your snake bites someone!