Elite Trainer Mark on X: Onix was Brock's first ever Pokémon. However, it didn't even evolve under his ownership, it evolved into Steelix while his brother Forrest was taking care of it.

Brock's Steelix, Pokémon Wiki

Brock's Onix Gym Heroes 69/132 Values - MAVIN

Onix - Azurilland Wiki

Magmaster12 Re watches the Pokemon Anime - Victory Road

Brock's Steelix, Pokémon Wiki

Brock (anime) - Bulbapedia, the community-driven Pokémon encyclopedia

How to Evolve Onix to Steelix on Pokemon Leafgreen/Firered

I'm making a Pokemon Game where you fight the Gen 1 gym leaders backward but also changed thinking about reversing the Elite 4 order as well and giving them dual typing to

Elite Trainer Mark on X: Onix was Brock's first ever Pokémon. However, it didn't even evolve under his ownership, it evolved into Steelix while his brother Forrest was taking care of it.

Onix, Pokémon Wiki

Elite Trainer Mark on X: Brock's starter Pokémon was an Onix that was given to him on his 10th birthday by his father, Flint. I'm not sure what other Pokémon Flint would have given his son considering they lived at a Rock-type Gym, so I guess Onix was pretty

Brock's Steelix, Pokémon Wiki