Myulan isn't human, or at least she isn't anymore. At 6:44 in eps 28, she describes herself as a majin.

What Shizu was turning into through Ifrit's influence, though she notably died as a human by holding Ifrit off for as long as she did. I think that little shit that got eaten by the Orc Demon Lord was one as well.

Unnamed monsters also are drastically weaker than they are once they're named. Aside from Treyni and Gabiru (though Rimuru overwrote that for him, haha), Rimuru named everything in the forest personally, and Veladora named Rimuru (giving him a boost).