Shiva Purana
चण्डरश्मिस्तु मार्तण्डो मोहितश्शिवमायया ॥ कामाकुलो बभूवाशु दृष्ट्वाश्वीं हयरूपधृक् ॥ २१॥
The sun of fierce rays, deluded by Śiva’s Māyā became lustful on seeing a mare and assumed the form of a horse.
english translation
caNDarazmistu mArtaNDo mohitazzivamAyayA ॥ kAmAkulo babhUvAzu dRSTvAzvIM hayarUpadhRk ॥ 21॥
hk transliteration by Sanscript