Theseus and Hippolyta, queen of the Amazons (or of her sister Antiope), with whom Phaedra fell in love. Apart from the obvious connection with his mother's name, the meaning of the name ('horse releaser', as for his mother) is specific for Hippolytus: while riding along the seashore his chariot horses, frightened by a huge bull that reared up out of the sea, threw him out and dragged him to his death.
Perhaps in view of this we should see his name as more 'loosed horse' than 'horse looser'. At the same time, even if he had not been killed in this way, Hippolytus would have been a fitting name for a charioteer, who 'releases horses'. Another character of the name was a giant, overcome in the battle of the gods against the giants by Hermes.