In addition to the examples presented by the first Educator, this poem also contains the devices listed below.
Allusion: The nightingale makes an allusion (or reference) to a famous classical composer, Wolfgang Amadeus Mozart: "And you are Mozart in disguise / Come to earth before my eyes". This reference functions to highlight how naive the nightingale is, that she genuinely believes the frog possesses the same level of musical genius as Mozart. It also further anthropomorphizes the animal characters in this poem, in that they are cultured enough to be aware of famous human composers.
Anthropomorphism: As mentioned in the last term explanation, the animals of the poem are anthropomorphized. This means that they are endowed with human qualities and behaviors. From the frog selling admission to the nightingale's shows, to the comical regal titles given to the other animals, the characters in this poem are anthropomorphized at every turn.
Anastrophe: This is a literary device in which the typical order of words are switched. Rather than the sequence of "adjective noun," the author chooses "noun adjective." In this poem, the speaker states "And the frog observed them glitter / With a joy both sweet and bitter." The poet chose to write "a joy both sweet and bitter" rather than "bittersweet joy" to draw special attention to the line. The reason for this is that this line is one of the first indications that while the frog reaps the rewards of the nightingale's song, he is still not satisfied. Something sinister lingers under the surface in this "sweet and bitter" feeling, and the speaker wants us to make a special note of it.
Rhyme Scheme: This is the pattern of rhyme within the poem. It is true that the poem primarily consists of paired couplets, as the first Educator mentioned. The rhyme scheme for the majority of the poem is "aa bb cc." This rhyme scheme is pleasantly musical, it is easy to read. It, in a way, invokes the singsongy tradition of storytelling—and this poem certainly frames itself as a fairytale.
Archetype: An archetype is a concept which has been used time and again, over and over, because it works so well. Essentially, it is a prototype. For example, the fairytale is a prototype of storytelling; we could say it is an archetype present in this poem. The text provides a great deal of evidence for this, but most telling of all is the first, immensely recognizable line: "Once upon a time..."