The Southern Europeans of Italy and Greece believed that this was caused by Fauns and Satyrs.
Spirits of the wild that are often confused for the other, they are often seen in the company of nymphs, Dionysus, Pan, and other gods. Satyr appear as hairy humans with horse or goat ears and tail, The Faun have the lower bodies of bipedal horses or goats and often have horns. Both love music, alcohol,sex and are traditionally shown with rather painful looking erections. Females were later created by poets.
Famous Satyr and Fauns are
Faunus, Roman god of forest, plains and fields; when he made cattle fertile he was called Inuus.
Fauna, Roman goddess of the same things Faunus is.
Silenus, Dionysus' tutor and friend. King of Nysa. He usually has horse-like features and is depicted as older than other Satyr
Masryas, A faun who challenged Apollo to a music contest and after loosing was flayed by the god for his hubris.
In popular culture there are
The Men of Leng, created by HP Lovecraft
CS Lewis' Mr Tumnus
Grover Underwood from the Percy Jackson series
Phil from Disney's Hercules