Jasmine had an Onix, who had lived 100 years to evolve into a Steelix.
Jasmine used her Steelix, which looked like an Onix, against Brock and his own Onix. Onix used Bind on Steelix but it was hurting itself, causing Steelix to reveal its true look. Steelix used Crunch, which knocked out Onix, and Jasmine revealed that Steelix used to be an Onix too before it evolved because it was really rare to see a Steelix.
|+ indicates this Pokémon used this move recently.*|
- indicates this Pokémon normally can't use this move.