Rattle snakes are animals which spawn in the desert, but often roam into other biomes. An attacking snake will result in a snake bite, which if not treated (with sterile wool pad and antivenom knife) will result in death. Snakes can be killed using a knife, yielding snake skin which can be crafted into snake skin boots, or by running over with a Crude Car which does not yield snake skin.
Encountering a snake while on a riding horse will kill the horse, from which antivenom can be extracted using a knife and a clay bowl. A Snake encountered while riding a horse-drawn cart will result in a tipped horse-drawn cart which can be righted using a rope (which is consumed).
Note: To kill a snake, you must be standing on the tile next to the snake. Then, carefully, while holding a knife, click the snake while it is highlighted. Be careful not to step on the same tile as the snake, and watch out for when it moves. Stand on tiles containing items to prevent it from attacking you while moving.