star (white patch on a horse's forehead)
Equestrian term for a white marking on a horse's forehead, resembling a star.
その馬の額には、はっきりとした星月があった。
That horse had a clear star on its forehead.
Compound of 星 (hoshi, 'star') and 月 (tsuki, 'moon'), referring to the shape of the marking. The reading ほしづき is a rendaku form.