Symbolism, Imagery, Allegory
There's probably a better way to describe this symbol, but how else do you fit together the massive snake jaws of male dwarfs and the wings of male sprites? This book may have a big female character—we're looking at you, Holly—but it's clearly a book made by and for boys.
Think about the special abilities certain kinds of fairies have. Sprites are the only ones with real wings, but you never see them mentioned on a female sprite—we're even told that if you "give a fairy a pair of wings [...] he thinks he's God's gift to women" (8.193). And what are wings useful for? Flying. And insofar as flying fosters freedom, independence, and the ability to escape tricky situations, we're gonna go one step further and say that wings—when it comes to sprites—represent male privilege.
Sprites aren't the only dudes with physical symbols of male privilege, though, and male dwarfs get in on the fun with their amazing—though kinda gross—dirt-eating skills (7.4). Mulch may be disgusting and also a kleptomaniac, but his tunneling teeth are what count—they give him access to places he otherwise couldn't get into, are his ticket to a reduced jail sentence, and generally facilitate his ability to maintain control over his existence (his tunneling ability ultimately enables him to fake his death, remember). If that's not privilege, then we don't know what is.
Maybe you're thinking these guys are all just born this way so their bodies can't actually symbolize male privilege. Maybe you're thinking it's not their fault. And maybe you're right. But the thing is, when questions of privilege come up, it's important to consider how the other half lives—in this case, then, we're talking about the ladies. And in this book, female characters spend most of their time locked up, passed out, or mentally checked out—even Holly, who's supposed to be "the strong one"—which means that dudes being able to get around definitely matters, and definitely puts them in a privileged position.
If this all irks you, then have no fear—we've got plenty to say about gender over in the "Themes" section.