The Mega Stone tag covers two types of items in the Pokémon series: Mega Stones, which are used by the Pokémon themselves, and Key Stones, which are used by the trainer. I assume the justification for this was that the two items were similar enough that separate tags weren't considered necessary. I used to think that reasoning was fine, but things have changed a lot over the past few months.
With practically every major character in Pokémon Legends: Z-A having a Key Stone somewhere in their design, the number of posts with the Mega Stone tag has skyrocketed since the game's release. As of the time I'm writing this, nearly two-thirds of all Mega Stone posts have the Legends: Z-A tag, with more than half of those being of Canari, Gwynn, and/or Emma, the three most-tagged Z-A characters. Searching for mega_stone isn't really that different from searching for pokemon_legends:_z-a *girl*.
Personally, I don't see much point in keeping both types of stone under the same tag. I do think there would be some genuine utility in a tag specifically for Mega Stones, as there are a decent number of posts which depict them. While it's not impossible to search for these kinds of posts as is, any search that tries to narrow things down will inevitably run into false positives or negatives. Mega_stone pokemon_(creature) would still turn up posts where human characters are present (Gwynn alone accounts for more than 100 of those results), mega_stone pokemon_focus leaves out a lot of posts due to the two tags having strangely little overlap, mega_stone no_humans would exclude posts of humanoid Pokémon, and mega_stone -pokemon_legends:_z-a would still include posts of characters from other games that have Key Stones.
Overall, I think creating a key_stone_(pokemon) tag and using mega_stone only for actual Mega Stones is the best solution here. I'd be more than willing to go through these posts and fix the tags myself (or sort them into favgroups and submit a mass update BUR), but I'll wait to see what everyone else thinks before committing to that.
Also, while this is only tangentially related, I have an additional question: when exactly does a post qualify for the mega_ring tag? I would just add them to any posts with characters who wear a Mega Ring, but there are plenty of examples where it's not immediately obvious that it's a Mega Ring. Take post #10154208, for instance. Someone not familiar with the character would just assume it's a generic bracelet, as there's nothing particularly noteworthy about it visually. If we're going by the principle of "tag what you see, not what you know," then these kinds of posts wouldn't qualify, right? Once again, I'm be willing to clean up this tag if needed, but I want to know what everyone else thinks first.
Updated by BKaz20