Well, Elvis's Mother thought it was a word! She used it right up to her death.This is very poor Latin structure. There is no such word as "Elvi." First declension Latin nouns ending in -us form the plural by dropping the -us and adding -i. Those ending in -is (I forget the declension) form the plural with -es. Thus, it should be "the flying Elves" (pronounced el-veez).