Wilhelm Albert Włodzimierz Apolinary Kostrowicki (1880-1918) had a Polish aristocrat as mother, and possibly a Swiss nobleman as father. Allorge records the speculation that his father was an Italian bishop, perhaps the bishop of Monaco.) He was born and brought up in Monte Carlo, and studied at Nice and Cannes, but moved to Paris when he was eighteen, and took the name Guillaume Apollinaire. He was a poet and writer, and a close associate of modernist writers and artists. His most famous work is the play Les mamelles de Tirésias (The Breasts of Tiresias, first produced 1917), inspired by Greek mythology, which imagined a woman called Thérèse, who changed her sex in order to establish power over men and to bring about equality between the sexes. In 1917 Apollinaire wrote programme notes for a ballet by Erik Satie, in which he coined the word “surrealist”. He wrote a number of short stories which adapted classic fantasy characters, including Merlin, Arthur, and the Wandering Jew: these were mostly collected in L’Hérésiarque et Cie (1910), which was translated by Remy Inglis Hall and published as The Heresiarch and Co in the USA (1965) and as The Wandering Jew and Other Stories in the UK (1965). For more information see the Encyclopedia of Fantasy.
He is perhaps most famous for his espousal of the cubists. He wrote a preface to the catalogue of the first cubist exhibition outside France (Brussels in 1911). He was arrested in 1911 for being implicated in the theft of works of art from the Louvre (he had briefly sheltered the perpetrator). He did not only promote the work of modernist painters, but also of the naive painter Le Douanier Rousseau. (It was Apollinaire who brought Rousseau to the celebrated but joky banquet in his honour, organised by Picasso in 1908; and two years later he wrote the poem that would be inscribed on Rousseau’s tombstone in Montmartre.) He was also among the first to defend the writings of the Marquis de Sade: he himself contributed to the genre of pornography, writing The Eleven Thousand Rods and The Exploits of the Young Don Juan (both translated for Olympia Press in the 1960s).
He was wounded in the Great War, and never recovered his health; he died on 9 November 1918, in the course of the great Spanish Flu epidemic.
My information comes so far from Wikipedia and from H. Allorge’s entry in the Dictionnaire de Biographie Française, vol. 3 (Paris, 1939), 122-124.