This is according to the Pokemon Species.
- Dunsparce (Land Snake Pokemon)
- Ekans (Snake Pokemon)*
- Onix (Rock Snake Pokemon)
- Steelix (Iron Snake Pokemon)
- Servine/Snivy (Grass Snake Pokemon)* *
- Seviper (Fang Snake Pokemon)
Source
*= Arbok may also be considered a snake
* *= Serperior may also be considered a snake
Trivia: Ekans is 'snake' backwards, and Arbok is 'kobra' backwards.