While on the topic, I did look through the new and popular section on Itch to see if any other categories came to immediate mind, and I noticed a few common categories that should probably be elevated, possibly made official in some capacity, or at least just simply aliased together for search result's sake:
"hide-and-seek" - 74 results
"hideandseek" - 14 results
"hideseek" - 5 results
"hide" - 45 results
^Has slightly different connotations than just "stealth"
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"femdom" - 233 results
"female-domination" - 31 results
"femaledomination" - 2 results
"dominatrix" - 6 results
"domme" - 1 result
^Just a quick example of another category with multiple hundreds of games that has a split tag argument. Happened to notice because its often linked with the monster-girl example in the OP.
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"monkey" - 327 results
"monkeys" - 42 results
"monke" - 78 results
"gorilla" - 90 results
"gorillas" - 4 results
"ape" - 22 results
"apes" - 2 results
"chimp" - 10 results
"chimps" - 2 results
"chimpanzee" - 1 result
"chimpanzees" - 1 result
"orangutan" - 0 results
"orangutans" - 3 results
"primate" - 1 result
"primates" - 1 result
^common genre especially for memes or jokes. "Ape together strong" this would be a big category if all of it were merged.
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I noticed many more smaller examples where hyphens or singular/plural forms split some very niche tags, but the above are just examples that cross over into the 100+ titles count and need a little bit more than just hyphen or plural fixes.