In the Pokémon TV series, Jessie’s snake-like Pokémon partner is Ekans.
Jessie receives Ekans in the original series and commonly uses it alongside James’s Koffing during Team Rocket’s schemes. Ekans is a Poison-type Pokémon whose name is “snake” spelled backward, a naming pattern also used for several Pokémon.
Ekans later evolves into Arbok. Jessie’s relationship with Ekans is one of the earliest examples of Team Rocket members caring about their Pokémon despite their criminal plans and frequent defeats.
A common mix-up is answering Arbok, because Arbok is the evolved form most associated with Jessie in later episodes. However, the Pokémon Jessie originally has is Ekans, while Arbok comes after evolution.