Mewtwo & Mew GX (Unified Minds) Tag Team GX cards were one of the most popular, most creatively ambitious, and most collector-friendly mechanics ever introduced in the Pokemon TCG, combining two Pokemon on a single card with powerful combined attacks, elevated HP totals, and artwork that told a story about the relationship between the featured Pokemon
Either way, the format was built around the five Elder Dragons cards from 1994
The great thing about Pokmon cards is that the collectors decide what becomes the most valuable based on how much they love the card
This gets messy when considering cards that only add one additional counter, like Pir, Imaginative Rascal
It's impossible to tell how long they've been there, and, even more scarily, we don't know what's put them in that state