Yeats is one of the most important poets of the period we call Modernism. Yeats's poem, like much Modernist poetry, takes inherited forms and meters and gives them a thorough makeover. So, for example, the meter of Yeats's lines is all over the place; some lines have five syllables, some have six, others have eight, others have ten (see "Form and Meter" for more on this).
In addition to a pretty variable meter, Yeats takes a very traditional four-line rhyme scheme (found in the classic ballad), and adds two lines. Those two lines are kind of like the new haircut you see somebody get on one of those makeover shows. The metrical irregularity of "The Wild Swans at Coole" and its revision of a very old form are distinctive features of Yeats's style. Take a look at some of his other poems, and we guarantee you'll find some of the same stuff!