It doesn't make sense. Shouldn't it be something like a normal type move?
Gamefreak decided to make it a Poison-type move. Similarly, they originally made Curse a ???-type move, but later changed it to a Ghost-type move. They also made Charm into Normal-type, but, in Gen 6, it became a Fairy-type move.
Anyway, Coil is a more common move for most Poison-types or "snake-like" Pokémon to learn.
Coil is a move that really reminds you of all the snake Pokémon. After all, snakes are serpentine creatures that can coil themselves up like a slinky. And what are snakes known for? Being venomous. Poison Types, are, well, known for being venomous or poisonous. A lot of snake Pokémon can also learn this move, and some snake Pokémon are even Poison-Types! So really, once you think about it, it makes a lot of sense that Coil is a Poison-Type move.