I have gone out, a possessed witch, haunting the black air, braver at night; (1-2)
Why might our speaker be braver at night? We get the sense that she enjoys being outside of the hustle and bustle of everyday life. It's when she's alone that she can allow her real identity to come out.
dreaming evil, I have done my hitch over the plain houses, (3-4)
Notice how there don't seem to be any fellow witches in this picture? Our speaker's out there on her own, above all the houses where everyone else is doing their living. Describing the houses as "plain" suggests that they're both normal and, well, boring.
lonely thing, twelve-fingered, out of mind. (5)
Yup, lonely and isolated are pretty much the same thing. You don't get much more explicit than this, folks!
A woman like that is misunderstood. (13)
Lonely, misunderstood – quite frankly, our speaker is a poster child for loners everywhere. Notice how the phrase isn't even "women like that are misunderstood"? Even when she's an example, she's all alone.
I have found the warm caves in the woods, (8)
Funny that caves in the woods seem a whole lot more appealing than the "plain houses" of the first stanza. We get the sense that our speaker has to find new places to call home – ones off the grid of the rest of the world.
fixed the suppers for the worms and the elves: (11)
Not exactly the party of the year, eh? The fact that our speaker doesn't hang out with humans could mean that she's forming alternate communities – or it could suggest that she's trying desperately to find something to connect with.