A genre that has other genres under it.
Typically a genre can be considered a metagenre if it isn't really a genre at all - it's more like a "general style" than "genre".
Think of anime. Anime isn't really a genre of its own. There are many sorts of anime movies that have the sort of style, but like any other art, it can (possibly) still be boxed into perfectly ordinary genres. Patlabor is mech sci-fi. Mononoke is clearly fantasy. Same for furry art - both Disney's Robin Hood and N64 game StarFox 64/Lylat Wars have foxy heroes, but the other is fantasy and the other is clearly sci-fi.