How do I encounter the last monster in the CORE/Hotland?

I’m doing the genocide route on my PS4, and I’ve cleared both hotland and the CORE, however there is one more monster left and I can’t find them, I’ve backtracked through the entire map (Hotland and CORE) at least 3-4 times, it’s been almost three hours and I still can’t find them. Is this a bug? Or do I have to look in a certain area?

(I first started off in Hotland, I got around a dozen or so kills and then got the rest of the kills in the CORE, have I missed one in Hotland or something?)

It's too late. Once you fight Mettaton NEO, you are locked into whatever ending you have gotten (unless you have a save from before the fight, in which case you should load that save and try again). No more enemies can be randomly encountered in Hotland or the CORE (or anywhere else in the game, for that matter), and you are now going to get the worst neutral ending instead of the genocide ending.

Next time, make sure you completely deplete the kill counter before fighting Mettaton NEO.