Someone who flies by the seat of their pants. The opposite of a "Planner" (aka "Prepper") on the NaNoWriMo.org boards.

Or as the "Killers" would sing:
"Are we preppin' / Or are we pantsers?"

Sorry, couldn't help it

...
Regards,
Eric Fretheim
Assistant Prep Leader, 2015 October NaNoWriMo Prep Challenge

"It is perfectly okay to write garbage-- as long as you edit brilliantly." ~C.J. Cherryh

“No, writing 50,000 words in a month is normal. You are not crazy. This is not insane.” ~Teri Brown