The minotaur was a specific monster that meant a specific thing and had a specific context. Separated from that all we have is Cow Man, a seventh level fighter with some extra crap tacked on. But that is ok. Our games can take a race of Cow Mans because we can all do something cool with that visual and put together a race of monsters that talk like Brad Garrett and Patrick Warburton.
Medusa is even more specific and the stories surrounding her are more interesting. It's almost a shame to remove her from that, like encountering A Cain or a pack of Herculeses. But man can you do stuff with the idea of an entire race/subrace of these people. Guy medusas, the petrification ecosystem, where they all came/come from...as long as you remember their blood makes weird horses then I'm fine with that and fuck Euryale anyway.
There are Magic-Users in this game but they made gingerbread cottage June Foray EE-HEE-HEE-HEE witches a separate monster.
So why, in monster manuals bloated with 14 different kinds of vampires, far too many interchangeable reptile men, mermaids, DOLPHINS, WATER WITH A MEANY FACE ON IT, and JUST REGULAR WATER all statted up as off the shelf options to murder your players, how absolute gold like the Kappas gets lost in the shuffle?
----Kappas are a lot of what I love as a DM: they have a fairy tale logic to dealing with them, they're slightly goofy, they're off the Kobold-Goblin-Orc ladder, they're gross and creepy, they afford all kinds of fun atmospheric horror and gore opportunities, they imply the world is way weirder at all times and ground the spirit world as a real and present concern, they have structure and institution and culture and HOBBIES and everything built into them, and they're slightly cute. I just love monsters who make players go "Hey that's weird and cool haha wait are we DYING oh fuck what the how do we kill it run away run away." They've even got all kinds of varieties you can play with from different folklore, even Oriental Adventures gives you two.
Like ghosts, they are geography in a way that even Dracula can't compete with. Dracula has an address, just stay home, but if there's a shitty pond around your life could be endangered. They're also versatile. You can find all kinds of behavior from outright horror shows to mere mischief to the spirit of honorable wrestling in tales of the Kappas so that they can fill many different roles in a dungeon map or provide several kinds of encounters. They come alone or in bunches. They don't hoard treasure as much as some monsters but they hoard "dead adventurers at the bottom of a lake" and adventurers usually have swag.
They also pass the quick and dirty monster manual test of "would Hellboy kick this thing's ass."
----I've used Kappas before. My party's first encounter with Kappas was during a torrential rainstorm, threatening a town at the behest of an oni (disguised as a kappa) who was tricking them. Some of them weren't sure what to think of these things but the rest of the group filled them in. Which is why everyone was surprised when the kappas got back UP, because their head bowl refilled itself within two rounds in such heavy rain. So then it became a game of luring the kappas indoors to properly defeat them WITHOUT further endangering the people hiding inside those same buildings. It was a fun, brutal night, and one of my favorite session ideas I've run.
The next time they met a kappa it was a gluttonous, gambling fool who competed against the party in a race across the city that turned into an escape across the city, which WOULD have been one of my best-of sessions if I was the DM I am today back when I ran that. He got killed by a half elf pirate queen who was sponsored by space jail. Yes, I've used kappas before, almost as early as I'd begun running games, with the energy and inelegance of unbridled enthusiasm.
For those kappas, every kappa was one species and they changed over their life. the little yellow ones were like young nymph forms, the big hairy fuckoff scary ones were like a disease old kappas could catch where they went mindless and got awesome, like Gamma Alzheimer's, so on. Their brain was actually layered on the inside of their skills like a motherboard and they needed the tissue fluid in their heads to act as a cooling agent and conductor for their synapses. There's a membrane that connects their heads, though, like when you see a plastic skull that opens? And that membrane degrades as they age, so that by the time they're adult it tears away and they're fully exposed. They usually then invert that bowl and use it to help keep water inside their kollow skulls, and dislodging that bowl spilled their water and killed them.
This is called overthinking a monster. I'd been a DM for like 3 months at this point, so what excuse D&D has after multiple decades for not being able to leave Awesome enough alone, I have no idea.
When you staked a vampire you were literally nailing him to the fucking box. That's why that worked and dealt with a vampire. Similarly a kappa losing their water leaves them helpless, but they can always come BACK. They're in some ways a trap monster, also one of my favorite kinds of monster.
----I would allow kappa PCs. They'd work like how you think, claw attack, swim speed, water breathing, 1 pt AC bonus, infravision. I'd leave all the bewitching and vampiric stuff for NPC kappas. I WOULD however be permissive about what fluid can fill their head so their fellow PCs can reactivate them easily. Blood, urine, beer. I would let them carry a magic potion in their head, benefiting from its enchantment, until they let someone else drink from the potion. Then either their head is empty so they can be easily dealt with or, if there's still potion left, it becomes poison so they HAVE to dump it out themselves. You can only play a kappa if one of your other PCs has wrestled a kappa naked and won.
I don't feel like you're supposed to run into 12 different kinds of frog monster in DNDish games, and then each kind has another 14 types within that, and they're all effectively just frog monsters to Nongnards. I feel like frog monster, snake monster, lizard monster, are all options, like what fits your world? And now all th creature creep we've seen means "Ok you decided frog guys, how complicated do you want your frog guys? How much lore do you want? How specific do you want it?" Bullywugs are remarkably free of baggage compared to a slaad, so you can use them more places, but they also come with fewer ideas than frog guy brought to the table so they're a less attractive option sometimes.
Anyway I can see eschewing all that and saying "How about kappas instead of frog/lizard/snake/turtle/whatever guys?"
The kappa has one problem, one big problem, for a specific kind of player, and that's that Mario kicks their ass all the time. If you're one of the people who just realized that's why he fights turtles who die when they flip over, ruled by a giant fanged turtle monster, you're welcome. If you can't stamp out thew Nintendo references then any creepy mood you want to make with these guys can be kissed goodbye. At that point I suggest just trying to murder people with a wandering kappa pack.
Strangely I feel like 5e could work for kappas in a way that previous editions have been unable to adequately translate, and I solemnly vow that if I ever get to run a 5e game I actually made up myself then I'll come back here and post my kappa statblock for everyone.
I may drop a few into one of my Deep Carbon Observatory sessions.