I was trying to catch terrakion but I accidentally 1hitkoed it can I still catch it or do I have to get it from the gts (I didn't think to save before so yes I'm an idiot)
You should be able to find Terrakion again. All you need to do is beat the Elite Four and it should re-appear at the spot it was in previously. This works for any other legends you may end up KOing as well.
You can still catch it, you just need to beat the E4 and Champion again, and it will reappear.
Hope I helped!