Zenith is a VR MMO that went through YC https://zenithmmo.com/. Multiverse isn't exactly an MMO, it's more of a tabletop RPG platform (think Roll20), but is also YC funded https://www.playmultiverse.com/. There might be more.

Not MMOs, but YC has funded smaller gaming companies over the years (the names of which I do not remember, which speaks to their relative success)

Machine Zone is relatively successful. They also funded Mino Games https://minogames.com/en/, which might not be "successful" on your terms, but they've stayed in business for 10+ years which is better than many game studios

A family member of mine played Game of War (owned by Machine Zone) and made friends with some of the highest ranked players. The biggest whales were dropping $5-10k a day on power-ups.

Ah, forgot about Machine Zone, that's fair.

Mino Games was the one I was thinking about, although that website appears to be out of date but yes their games are still active.