It is indeed a myth, I’m afraid. Tyranitar is actually more of a dragon-like pokémon, and thus, it’s more related to pokémon such as charizard, haxorus, and so on.
However, believe it or not, tyrantrum and all other dinosaur-like pokémon are actually ancestors of avians. It’s a long and complicated subject, actually, but suffice to say, the archen line serves as a sort of “missing link” in the evolutionary family tree between ancient dinosaurs and modern-day bird-like pokémon. So to answer your question in the simplest way possible, according to both fossil records and DNA evidence, the closest living relative of tyrantrum is not tyranitar but rather … torchic.
But more broadly, pretty much every bird-like pokémon is descended from dinosaur-like pokémon such as tyrantrum. Perhaps not entirely directly, but nonetheless.
Also, as an aside, it’s important to note that aerodactyl is not related to either of these (dinosaurs or birds). Rather, it occupies another genus altogether, which it shares with … no known species of pokémon, actually. It’s quite fascinating.