Pre-Vedic mythology had solar mare goddesses such as Saranyū and Urvaśi. Mare goddesses racing in myths (such as the Celtic goddess Macha or Rhiannon hint at this ancient link.
When patriarchal cultures claimed the sun as male, the solar mare goddesses became linked to the planet Venus. Venus as the morning star always rises before the sun and is just as brilliant. As a result the mare goddess also became more eroticized.