Twins

Twins are a pair of children that are born from the same pregnancy. Some gods, Titans, demigods, and mortals are twins. They don't need to, necessarily, look alike.

Titans

 * Prometheus & Epimetheus
 * Helios & Selene

Gods

 * Apollo & Artemis
 * Ares & Enyo
 * Hypnos & Thanatos
 * Deimos & Phobos
 * Pegasus & Chrysaor
 * Zethes & Calais - Boreads
 * Thaleia, Aglaia, & Euphrosyne - Graces (triplets)
 * Boreas, Notus, Zephyrus, & Eurus - Anemoi (quadruplets)
 * Calliope, Clio, Melpomene, Erato, Euterpe, Polyhymnia, Terpsichore, Thalia, & Urania - Muses

Demigods

 * Castor & Pollux
 * Zethes & Calais (they are immortals, but were demigods)

Monsters

 * Cottus, Gyges, Briares - Hundred Handed ones (Hekatonkhieres); triplets
 * Oreius & Agrius - Bear Twins

Mortals

 * Bobby Chase & Matthew Chase

Giants

 * Otis & Ephialtes