Twins

Twins are a pair of children that are born from the same pregnancy. Some gods, Titans, and demigods are twins.

Titans

 * Prometheus & Epimetheus
 * Helios & Selene

Gods

 * Apollo & Artemis
 * Calliope, Clio, Melpomene, Erato, Euterpe, Polyhymnia, Terpsichore, Thalia, & Urania - Muses
 * Deimos & Phobos
 * Alexiares & Anicetus
 * Pegasus & Chrysaor
 * Boreas, Notus, Zephyrus, & Eurus - Anemoi
 * Zethes & Calais - Boreads
 * Stilbon, Hesperos, Pyroeis, Phaethon, & Phainon - Stars
 * Thaleia, Aglaia, & Euphrosyne - Graces

Demigods

 * Castor & Pollux
 * Zethes & Calais (they were made immortal, but were demigods)

Monsters

 * Hydra, Orthrus, & Cerberus - Children of Echidna
 * Cottus, Gyges, Briares - Hundred Handed ones (Hekatonkhieres); triplets
 * Oreius & Agrius - Bear Twins