First thing I think of is Dick Cheney. Then I shudder.
The second thing is The Moon is a Harsh Mistress but that was just a name.
The first thing I thought of when I saw the subject line was Matthew Shepherd, but that would probably make a pretty shitty costume for a Girl Scout.
yellowstone national park (old faithful?) or devils tower.
A Wyoming costume could easily be made out of any large cardboard carton. Cut holes for head and arms, and voila! No need to paint–kraft paper brown is the perfect color! 
Actually, I like the buffalo idea, but if that’s too daunting, I’d go with a cowgirl (there is a cowboy on the license plate and the state quarter, as has been mentioned.)
Last in the alphabetical list of states? Not helpful, I expect.
Otherwise, lots of wide open spaces.
Cheap Devil’s Tower costume: pile some mashed potatoes on your head and sculpt them with a fork. Butter and Gravy optional.
I think a buffalo costume would be fun. You can definitely get some cool shaggy material from the fabric store.
I think of Yellowstone and buffalo when I think of Wyoming.
Rats! Missed the edit window!
Less messy idea: dress up as a jackalope.
[del]Not surprising, since Wyoming is The Cowboy State.[/del]
I see that Wyoming is no longer officially The Cowboy State but The Equality State. Jebus. The pussies have won.
A Devil’s Tower costume would be ridiculously easy to make.
Anymore? Rich people.
This one is so easy…
Get a Casper the Friendly Ghost costume.
Ta-da!
I guess Brokeback Mountain is out.
I associate Wyoming with The Grand Tetons. (Really.) I can think of a great Halloween costume, but not for a little girl.
She should wear a big (ie, larger than her torso) cardboard square/rectangle, because Wyoming is one of those states. Then she should have a mashed potato sculpture of Devil’s Tower on her head.
Jackson Hole, Jackson Pollock, Old Faithful.
You can’t get a costume out of it, but after driving across it last year…desolation. Really impressive, beautiful desolation. I am not being negative; it felt so big. I found it fascinating. The thought of crossing it in a wagon or something left me with even more respect than I’d already had for people who made that trip – when I moved my route roughly paralleled the Oregon Trail.
I see a lot of people from Wyoming who come here to ski, too, but that doesn’t help either.
Okay, I know I have a dirty mind, but that combination just sounds like a porn film to me.
I picture it as grey, which may be a holdover from our US map puzzle. (New York is orange.) I picture bison, no taxes, brown plains. I picture the boot outside the State museum.
http://i.pbase.com/o6/39/773939/1/86476133.weVrFtN7.PB20070821BootWYStateMuseum.JPG
Desolation, or emptiness.
They do have a huge rodeo every year called, “The Granddaddy of them All.”