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HP: 35 | ATK: 45 | DEF: 160 | SP.ATK: 30 | SP.DEF:45 | SPD: 70 | TOTAL: 385
This is definitely one way to min-max something. Onix is one hell of a defensive tank, that Defense stat is absolutely staggering. And it comes at the expense of basically everything else, except for Speed, which you kinda need to be a large rock snake. You can take physical hits for days, but can't really do anything back, nor can you really take any special hits, so even the weakest of those moves will take you out, due to the couple of x4 weaknesses. And there's the problem of Onix, it was designed to be the first major boss of the entire franchise, so its stats had to be carefully crafted so early trainers could beat it. So it can't dish out too much damage and takes a lot of damage from your brand new starters, aside from the shitty one.
But aside from that, Onix, or at least a Rock type, is great to use this early on, especially since it gains extra experience and gets STAB pretty early on. But you'll be focusing on the Ground typing more, since you can get better moves for it and when it evolves, you'll be able to rely on Steel STAB instead, but having a Rock move around is pretty useful. Might as well evolve it ASAP cause there's not much use in keeping it in its base form. But if you can get around the pretty sharp shortcomings, it's not too bad a Pokemon.