The complete Pokemon RBY pokedex, part 9

Pokémon Name: Onix
Type: Rock/Ground
Classification: Rock Snake Pokemon
Pokédex Number: 095
Ability: Rock Head and Sturdy
Dream World ability: Weak Armor
Useful Attacks: Double-edge
Location Found:
D/P/P/HG/SS: Oreburgh Mine, Victory Road, Iron Island
HG/SS: Union Cave, Mt. Silver, Ragged Cliff Gate, Victory Road, Rock Tunnel, Violet City (trade a Bellsprout to Kyle in house in southwest corner)
B/W: Relic Castle B7F

Onix sure is a cool looking Pokemon, but its usefulness is quite low. The defense stat is high but if you want to attack at all, and you do, this is not your Pokemon. Level it up a bit and then move along to Steelix, who manages to scrape up some attack power in addition to an outrageous defense.

Evolution: To Steelix via Trading with Metal Coat