Dragon's Crown is a fantasy side scrolling beat 'em up style game that came out in ~2010s, if you consider consoles like the PS3 and Xbox 360 retro.

It is similar to the old Capcom Dungeons and Dragons arcade game, which itself was similar to Golden Axe.

Well, it's fantasy (jrpg) and a real classic co-op - secret of mana for snes:
https://www.co-optimus.com/Editorial/1002/co-op-classics-secret-of-mana.html
https://www.nintendojo.com/features/columns/retro-scope/retro-scope-secret-of-mana

How has no one recommended Dungeons and Dragons: Tower of Doom and Shadow Over Mystara? Classic Capcom beat 'em ups with a ridiculous level of D&D added on top.

Cast a spell at the Beholder? Bad idea, unless it's looking the other way. Dwarves find extra treasure, there are tons of secrets and hidden items, the cleric can turn undead, it's a ton of fun. Available via emulation or as a 2-pack on Steam.

https://store.steampowered.com/app/229480/Dungeons_Dragons_Chronicles_of_Mystara/

With the tag of multiplayer retro fantasy, the first game that came to mind was gauntlet legends. Playing it co-op was a mind-blowing experience for my younger self.

How retro do you wanna go?

Divinity Original Sin, and the sequel, are older and both excellent

You might try Barony. There's for sure a learning curve, but it gives those retro arcade cabinet vides with the chunky 3d perspective and fast paced play style. Kind of like dnd if you weren't always pausing to check rules and you had a DM who hates you

Retro, coin-op, fantasy, co-op games that follow DnD classes, and first came to mind are:

Magic Sword

https://en.wikipedia.org/wiki/Magic_Sword:_Heroic_Fantasy

Dark Seal (AKA Gate of Doom)

https://en.wikipedia.org/wiki/Dark_Seal

Also, Magician Lord is a decent fantasy platformer that is listed as multiplayer(I only remember seeing it single player though)

https://en.wikipedia.org/wiki/Magician_Lord

[ Bubble Bobble is the GOAT of arcade coop. Look up the Power-Up cheat code, for input into the attract screen]

If you're open to expanding the horizons a bit, Tiny Tina's Wonderlands is a lot of fun. But it is a lot of mayhem and explosions and such. If you're familiar with Borderlands, it's that with DnD sauce.

Diablo on ps1 had a couch coop. Also legends of mana too.

Titan Quest is not RetroArch and more Action than RPG but it was a fun Diablo-like.

If you want something emulated, Golden Axe or Gauntlet Dark Legacy for action or Baldur's Gate Dark Alliance for action RPG.

Most of the Tales series. Phantasia, Destiny, Eternia, Symphonia, etc.

The old sidescrolling fighting games like TMNT on the NES/SNES were pretty good. Sticking with SNES, super mario kart, street fighter, and all important: NBA JAM!

Maybe Terraria?