MLA Citation for your version of Aristotle’s Poetics:
Aristotle. Poetics. Trans. S.H. Butcher. <http://classics.mit.edu/Aristotle/poetics.1.1.html#200>
This is an online copy of a text that’s in the public domain, so a lot of the typical MLA information (esp. for publication and copyright) is not available or does not apply. This doesn’t mean you can leave it out of your other citations, though!
This post doesn’t allow for the formatting, but be sure you use a “hanging indent” on all citations if the information runs from one line to the next. (Ex. Here, the web address should be indented further than the first line, because I need two lines for it to fit.)
If you have more than one source on your Works Cited page, remember to arrange them in alphabetical order, too!