-Well yes, Onix's evolution Steelix is better in every way you can see it. With a modified typing in Steel/Ground, Steelix gets to be neutrally hit by Grass type moves whereas Onix will most likely get KO'd in every single wa when hit by a grass type move. Even if Onix were to hold an eviolite, it is still not to be compared to Steelix. Steelix also has stellar defenses that make it almost impossible to break with a physical move. With a modified movepool and a decent Attack Stat, Steelix can now act very similar to Onix, just much better. Like most evolutions, Steelix is just much better to utilize over Onix and for this reason it is worth evolving Onix
