The sella turcica, a deep depression in the floor of the skull where the pituitary rests, is always translated from the Latin as "Turkish saddle", sella meaning saddle and turcica, "of the Turks". However, for the Romans during most of their empire, sella was a word for stool, not saddle. Indeed, neither the ancient Romans nor Greeks had a word for saddle because they didn't need one: they rode on blankets draped over the horses' backs instead.

While Greeks and Romans were busy perfecting their primitive riding techniques, the Turks and numerous Arab tribes developed and fine-tuned the saddle which, to facilitate rides of great distances, typically had high backrests (or cantles as they came to be called) and relatively tall anterior pommels. Following the introduction of the "European" saddle in the 4th century (called the sella equestris by the Romans) with its lower cantles and usually shorter pommels, the phrase "Turkish saddle" appeared, used by Europeans as a generic reference to any saddle with a high back and front.

Turkish horse saddle circa the 12th century; from God's Warriors: Knights Templar, Saracens and the Battle for Jerusalem, by Dr. Helen J Nicholson and Dr David Nicolle, Osprey Publishing, 2005

An 18th century Arab horse saddle, in the sella turcica style; from Spanish Arms and Armour, By Albert Frederick Calvert, J. Lane Publisher 1907

And so the rationale of using sella turcia in anatomy, which began appearing in medical texts in the 17th century: the saddle-like fossa of the sphenoid bone with a tall posterior "cantle" and anterior "pommel". It's not known who coined the phrase.

A sagittal section through the sphenoid sinus and sella turcica; image modified from Gray's Anatomy (1909) and Wikipedia.com