Jessie's Arbok

Jessie's Arbok is one of Team Rocket's Pokemon in the first season of the Pokemon anime (Indigo League). It first appeared as it's younger 'Ekans' form in the episode 'Pokemon Emergency. Jessie's Arbok was a birthday present as an Ekans for Jessie and the most loyal of her pokemon. Jessie's Arbok knows that Jessie does bad things but is so loyal as to not try to stop her. [1] Jessie's Arbok had such great a bond with Jessie as to evolve from Ekans to Arbok because Jessie wished deeply for it to. The last interaction Jessie's Arbok had with Jessie was seeing Jessie do good by holding Poachers off. Arbok was told by her to stay behind, be free, and protect Pokemon from the poachers of the region they where in. Thus Jessie's Arbok ceased to be Jessies' and in tears set out to do just that.

Source Citation

 * 1) Season 1 Episode 17 : Island of the Giant Pokemon