This is one side of a large block with decoration on several sides that now stands near the ticket office. A pdf describing Didyma has “Anta Capital of the Apollo Temple. On the immediate left hand of the landing the capital of the north-eastern anta of the Apollo Temple is presented which dates from the first half of the 2nd century BC. All three sides of the capital display a tendril woman with polos and wide-spread wings.” I then found in a Wikipedia article that speaks of a “Snake-Legged Goddess, often crafted by Greek artisans for the Scythian market, most of them depicting her as a goddess with snake-shaped legs or tendrils as legs”.
For years I did not know what it was, my thanks to people who thought along.