According to the Merriam-Webster online dictionary, the definition of an epigram is: "A concise poem dealing pointedly and often satirically with a single thought or event and often ending with an ingenious turn of thought."
The poem "Epigram Engraved on the Collar of a Dog Which I Gave to His Royal Highness" by Alexander Pope certainly fits this definition. It is definitely concise, as the title of the poem and the poem itself have exactly the same number of words. It is satirical, having a sharp barb at the end at the expense of the subjects of royalty. In fact, Pope was renowned and sometimes vilified for his satire.
The Kew in the poem refers to Kew Palace, which was a royal residence in the eighteenth and early nineteenth century. It's located west of London beside the Thames River. In 1738, when the poem was written, King George II, Queen Caroline, their son Frederick, and their three daughters lived at Kew Palace. Pope gave a dog to Prince Frederick, and the words of this poem were actually engraved on the dog's collar.