Some one edition creatures beginning with 'Y'. I am not entirely sure about all the yuan-ti because the names for the types of yuan-ti seem to change between editions.
Yag-Kosha (Supplement IV: Gods, Demigods, Heroes) [OD&D]
Yahoo (Polyhedron #106) [2e]
Yak (PC4: Night Howlers, GAZ12: Golden Khan of Ethengar) [BECMI]
Yaksha (Supplement IV: Gods, Demigods, Heroes) [OD&D]
Yallix (Dungeon #111/Polyhedron #170) [3.5]
Yau-Mor (OA5: Mad Monkey vs. the Dragon Claw) [1e]
Yeshir (Dungeon #106/Polyhedron #165) [3.5]
Yeshom (Savage Coast Monstrous Compendium Appendix) [2e]
Yihn-eflan (Windriders of the Jagged Cliffs, Monstrous Compendium Annual Volume Three) [2e]
Yitsan (MC9: Monstrous Compendium Spelljammer Appendix II) [2e]
Yphoz (WGA2: Falconmaster) [2e]
Yuan-Ti Fangblade (Monster Manual: Dangerous Delves) [4e]
Yuan-Ti Holy Guardian (Serpent Kingdoms) [3.5]
Yuan-Ti Ignan (Monster Manual IV) [3.5]
Yuan-Ti Mageslayer (Serpent Kingdoms) [3.5]
Yuan-Ti Sarpacala Malison (Dragon #370) [4e]
Yuan-Ti Wretchling (Secrets of Xen'drik) [3.5]
Yugoloth, Altraloths (Dragon Annual #2) [2e]
Yugoloth, Baernaloth (Planes of Conflict) [2e]
Yugoloth, Corruptor of Fate (Monster Manual IV) [3.5]
Yugoloth, Echinoloth (Stormwrack) [3.5]
Yugoloth, Gacholoth (Dungeon #49, Monstrous Compendium Annual Volume Four) [2e]
Yugoloth, Skeroloth (Fiend Folio ) [3.0]
Yugoloth, Voor (Monster Manual IV) [3.5]