> https://grammarist.com/usage/dwarfs-dwarves/ tells me it's a neologism or, perhaps, a neoarchaeologism? > popularized by Tolkien. The OED's only record of it (in a usage citation for "dwarf, n.") is "1818 W. Taylor in Monthly Mag. 46 26 The history of Laurin, king of the dwarves."