At least in Mexico, identical twins are called "gemelos", but fraternal ones are called "cuates". In standard Spanish, fraternal twins are called "mellizos", but in Mexico this word would be maybe synonyms with "gemelos". "Cuate" also means friend, and is of Nahuatl origin (from "koatl", snake or twin). It is interesting also that the most famous twins of the ancient Mexican mythology were Quetzalcoatl and Xolotl. Quetzalcoatl would be the precious twin (ketsalli + koatl), while Xolotl was the monstruous one, giving name to lots of things linked to ugliness or asymetrical pairs, like axolotl (water monster, Ambystoma mexicanum), texolotl (stone twin, pestle of the molkaxitl or stone mortar), wexolotl (old monster, turkey), xoloitskwintli (monster dog, the Mexican Hairless dog), and so on. Triplets here are called "triates", a mix of the latinate root "tri" with "cuates".