triolet prompt back in 1999 on Poets Online using that 8-line form.
Diane Lockward posted recently about a little contest for triolets at Allison Joseph's blog, The Rondeau Roundup. That's a blog that is self-described as being for "the exploration, appreciation and publication of the rondeau, rondel, roundel, rondeau redouble, rondolet, triolet, and ballade."
Want to give it a try? Look at our earlier prompt, Diane's post and Allison's contest.