Dog, Gun and "for to shoot" as they appear in a couple of lines were discussed a bit back.

A poet or lyricist often upends grammatical rules or tradition to fit the words to the line.
I took out my dog and my gun for to shoot all along the green banks of the Spey-ey-ey-ey.
The rhythm goes bumpity bumpity bumpity... The "for" has to be there to maintain the rhythm.