“The Tritonian Ring,” by L. Sprague de Camp

And what a lovely book front this is! It comes from the 1971 Paperback Library edition of The Tritonian Ring, by American science-fiction and fantasy writer Lyon Sprague de Camp (1907-2000), part of his Pusadian “sword and sorcery” series. Sadly, I don’t own this novel, so cannot judge it on any basis other than its cover. But a plot summary from Wikipedia reads:

When the gods resolve to destroy Lorsk, principal kingdom of the sinking continent of Pusad, because Prince Vakar, heir to its throne, is thought to be a threat to them, the king sends the prince on a quest to save the realm from destruction. Vakar is tasked with traveling the known world in search of what the gods most fear, accompanied only by his servant, Fual. He finds himself hampered by ignorance of just what that might be and continual attempts to murder him by parties unknown; meanwhile, his treasonous brother Kuros is plotting with the pirates of the Gorgon Isles, Lorsk's enemies, to overthrow their father. On his quest Vakar encounters Amazons, a seductive queen who is under a spell, an amorous centauress, sorcerers who command legions of headless warriors, and the dangerous Gorgonians themselves, masters of the medusas with their paralyzing glares.

I thought the willowy young woman shown above might be the “amorous centauress” in question; she’s clearly an essential character in de Camp’s yarn, being also represented on other versions of this book (such as this one). However, while said filly sports a tail, she lacks the horse’s aft end that I normally associate with such a mythical creature. Perhaps Midwestern artist David McCall Johnston decided that feature might not be universally appealing.

Johnston, born in South Bend, Indiana, back in 1940, painted a small number of SF covers. During the years he spent as a book-cover artist, he came to be known much better for his contributions to the fantasy-fiction field. You can enjoy more of his work here.