Post Pokemon that seriously aren't worth your time catching, and optionally the alternatives if you want something like it.
(Whether they're bad, you have much better options, or it won't help at all yet.)
Ariados(what Spinarak turns into) sucks. Its typing is helpful against fighting and grass, but its movepool is atrocious. Megahorn!... an egg move, X-scissor, a TM we can't get, Bug Bite, at a move tutor. For level-up moves, we have Pin Missile, which it learns at level 41. It learns Poison Jab... at LEVEL 50. When leech life gets buffed, it will be usable, but with one of two STABs and no good coverage, it's still bad.
Shadow Sneak(40BP) is too little too late, at level 19, and the same with Night Shade(level damage) at level 15. It only has Poison Sting and Leech Life early on.
Ariados is cool-looking and has alright stats for when you can get it, so this is pretty sad.(Not Reborn's fault, just it's canon movepool.) I bore with my shiny level 5 Spinarak with good-enough IVs up until it was a level 27 Ariados, at which point I looked it up and learned this. It became usable once it got Shadow Sneak and evolved, but I'm pretty disappointed in it. I sunk too much grinding time into it and it's just useless.
First gym it gets no advantage and is not good enough yet(level 22 to evolve), and it's not actually good against the second gym leader.
(Bug moves mean it will only get 1/2 resist, same as a mono-poison type, and Cradilily's rocks will crush Ariados flat.)
Avoid catching Spinarak at all costs.
Get a Poochyena if you want a physical attacker.
Get Gulpin and/or a Trubbish for your poison type(s). Poison resists the poison you see in the first city's back alleys, can't be poisoned, resists the grass and bug that you'll be seeing before and after the second gym, won't be squashed by Cradilily, and won't be slashed apart by the Pidgeys and Pidgeottos in Jasper. Looking at all of this, it's extremely helpful. It's a good defensive type, and two layers of Poison Spikes gets you a free Toxic on a whole gym leader's team(unless they are ground-immune). Trubbish gets Toxic Spikes, Gulpin gets Yawn. Both get Acid Spray, which means you don't have to worry when they use a potion.
For a bug-type, and a good Pokemon overall, get a female Combee.(Becomes Vespiquen. You'll need something with Headbutt to get your female Combee.)