I'VE GOT A NEW POKEMON I'M THINKING OF!!! And you're NOT guessing it this time!

Possible for rival huh? Why don't you double check it's on a rival's team. I know it can't be Kanto(Blue + Trace) and Sinnoh(Barry) because you've already said so.

That leaves Silver(Johto), May/Brendan/Wally(Hoenn), Cheren/Bianca/Hugh(Unova), Serena/Calem(Kalos, ruling them out though because you hate Serena), Hau/Gladion(Alolan), Hop/Bede/Marnie(Galar) and Nemona/Arven/Penny(Paldea). Rest don't qualify or were eliminated.

Silver has Gengar, Magneton and Alakazam(all ruled out because it's not Kanto), Sneasel(has regional variant in Hisuian so can't be that), Crobat(possibly) and Johto starter that beats yours(you dislike Johto so highly unlikely) but two of the three Johto starters can Mega Evolve(leaving Cyndaquil who is not on your most liked list). It's index number is Prime though so that eliminated it(leaving just Crobat).

May/Brendan share a similar(if not identical) team in starter that beats yours with Wailord/Magcargo/Breloom being the Water/Fire/Grass alternates to go with Swellow and Raichu. You ruled out Kanto so Raichu is out and you mentioned they can't Mega Evolve so that rules out the starters. Swellow index number is prime so that leaves the alternates.

As for Wally, his team has Altaria, Delcatty, Roserade, Magnezone and Gallade. Gallade and Altaria can Mega Evolve, so that leaves the rest(you like Delcatty alot).

As for Unova, got Cheren and Bianca in the base games and Hugh in the sequels. Their teams vary based on your starter pick but since the Pig can Mega Evolve and Oshawott has a regional variant, only Serperior is a possibility with respect to the starters. The rest of Cheren's team is Liepard, Elemental Monkeys that match your starter type, Unfezant, Gigalith and Haxorus. Unfezant is eliminated from index being prime.

Bianca's team has Elemental Monkey that beats yours, starter weak to yours, Musharna and Stoutland. Her Post-Game team adds Mienshao and Chandelure. All of them are possible save for the starters I ruled out.

Hugh's team mirrors Cheren with Unfezant, Elemental Monkey weak to your starter and starter that beats yours but has Bouffalant, Eeelektross and Flygon(which you ruled out). Eeelektross can Mega, leaving just Serperior, Bouffalant and Elemental Monkey options.

Ugh I'm getting drained going over this. Is it anything post Gen 5 to narrow it down?

typhlosion??