In line 19 we can see that "Coursers" are mentioned and are explicitly compared to something else. Whenever you have to answer a question like this that requires you to focus on specific words or phrases rather than the poem as a whole, it is vitally important that you look at the words in context and try to deduce the meaning from the stanza in which they come. You will find that, using common sense, you can work out a lot of the answers to such questions by using such a method. Let us apply this method in this case:
More rapid than eagles his Coursers they came...
Note here how the "Coursers" (the reindeer) are compared to eagles in their speed at answering the call of St. Nicholas. Thus it is that the "Coursers" in this part of the poem are compared with eagles, emphasising their speed and quickness.