Boss Monsters are a category of creatures from Undertale. In the lore, they can only age when their offspring ages and their soul can linger after death. This is as much info that is concrete about boss monsters. It's not even certain whether all boss monsters are goat-like, or if we're just meeting one family of boss monsters who happen to be goats. "boss monster" is a category to explain characters in the Undertale story, and I'd struggle to call the term an actual species. And yet, the boss monster page here seems to have dipped a bit too far into the fan juice.
They are creatures with a mix of caprine and lion-like features such as fangs, paws, droopy ears, and horns when matured. Similar to real life lions, they appear to display sexual dimorphism, with males often having manes.
...Lion? Where? Because Asgore has sideburns and a beard? All sprites of the boss monsters in-game look like anthro goats. boss_monster solo caprine has over 84 pages of results. boss_monster solo felid has less than 1 page. I think post #2214741 is the only one that actually resembles a caprine/feline hybrid, and even that also tags canine for... some reason? Everyone draws them as goats, they look like goats, please can we just call them goats in the same way that we call Alphys a lizard or Noelle Holiday a reindeer.
They also appear to have unique magical powers of sorts, often dealing with or themed around flames or fire
Isn't Toriel the only one with fire-related powers?
The page is trying so desperately hard to justify the tag's existence, even when it itself admits that the only three Boss Monsters are Asgore, Toriel, and Asriel. And even still, the tag ends up on posts showing Flowey the Flower (who is Asriel in another form but looks completely different), or Ralsei (who exists in a separate universe where boss monsters might not even be a thing), or Muffet (who is a boss in the Undertale game but not a "Boss Monster"). The term is rife for confusion and mistags. Honestly, I'm surprised it's not mistagged on any non-Undertale posts, given how generic the term is. If there's concern for fan characters losing searchability, there's no reason undertale fan_character caprine wouldn't hit the same beats.
I'm starting this as a tag discussion at first in case I'm missing something obvious, but I'm very tempted to throw the tag into a BUR and do something to it. Maybe make it an invalid tag (due to genericness), maybe alias it to Undertale, the details can be hashed out. I just don't think there's value in a tag that can only ever refer to three specific characters in a specific context (Undertale, not Deltarune) and a small spattering of fan characters who just look like goats anyway.
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