by 1995, from horn (n.) in the sexual sense (see horny) + dog (n.).
horn
hornbeam
hornbill
hornblende
horn-book
horndog
horned
horner
hornet
horniness
hornless
)window.location='https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=byronic106@yahoo.com&lc=US&item_name=Donation+to +Help+Keep+Etymonline+Free+and+Open&no_note=0&cn=&curency_code=USD&bn=PP-DonationsBF:btn_donateCC_LG.gif:NonHosted'){kind=link}