Believe it or not, poems don't spring fully formed from an author's brain. Check out the writing and revision process that Owen went through with "Dulce et Decorum Est" here: you can scroll through four different versions of the poem (in manuscript form) and see just how the masterpiece we read today was created.
Apparently, 19th century university students changed Horace's quote into a drinking chant:
"Dulce et decorum est pro patria mori,
sed dulcius pro patria vivere,
et dulcissimum pro patria bibere.
Ergo, bibamus pro salute patriae"
Roughly translated, that's: "It is sweet and proper to die for one's country, but it is sweeter to live for one's country, and it is the sweetest to drink for one's country. Therefore, let us drink to the health of our country." We can't verify this, but you can follow up on it here.