In a poem about hunting a deer, wildness is a given. Every time the speaker refers to the "hind," we think of a wild animal that could flee at any moment. Wildness in this poem is a metaphor for the fact that the deer woman the speaker is chasing cannot be caught, or isn't meant to be caught. She's metaphorically wild, at least for the speaker, even though she seems tame.
Lines 5-7: The hind keeps fleeing. Well, that's natural: this is what wild animals do. Flight here is a metaphor for the fact that the hind is not to be touched or shouldn't be pursued. The speaker's hunting of the hind is a metaphor for the process of courtship or sexual pursuit. Note all the alliteration here as well, with those M and D words.
Lines 7-8: We have several metaphors to note here. The hunting metaphor is on display again, this time in the form of a net. "Wind" too is a metaphor that emphasizes the hind's wildness or un-tame-ability (is that even a word?). Note also the alliteration of the S words ("since" and "seek").
Lines 13-14: The writing on the hind's neck says she seems tame but is actually wild. Okay, got it—thanks for the heads up. Here wildness is a metaphor for the dangers of capturing this "deer." She's not wild in the sense of sexually crazy, or likely to run away really quickly. No, no, she's wild in the sense that she could hurt you, or rather touching her could cause you a lot of pain.