The couplet, as stated in the question above, is a poem that presents successive lines of similar metrics, rhyming with each other and presenting basically the same number of syllables. Generally these poems have at least two lines of poetry that rhyme and may not have a limit on the number of lines. This type of poem has become very popular in English-speaking countries, as it gives more freedom for accommodation when it comes to the number of lines, but they have some difficulty regarding the number of syllables and the metric between the lines.
Answer: The cherubin were angels, and in Macbeth's simile the couriers were not blind but invisible horses evidently coming to exact revenge for the murder. The word "horsed" shows that the "sightless couriers" are invisible horses.
