A quick look through the 'I's gives me the following list. I haven't checked this one very thoroughly, so there are bound to be a few that have appeared in other editions.
Ibis (Races of Faerûn) [3.0]
Ibixian (Monster Manual III) [3.5]
Ice Lizard (MC14: Monstrous Compendium Fiend Folio Appendix) [2e]
Ice Serpent* (Dungeon #27) [2e]
Ice Serpent* (Monsters of Faerûn) [3.0]
Ice Warrior (Dungeon #159) [4e]
Icegaunt (Frostburn) [3.5]
Icetail (FR14: Great Glacier) [2e]
Icy Prisoner (Dragon #324) [3.5]
Id Fiend (Monstrous Compendium Dark Sun Appendix: Terrors of the Desert) [2e]
IffanbutT (ST1: Up the Garden Path) [BECMI]
Iguana, Horned (MC5: Monstrous Compendium Greyhawk Appendix) [2e]
Iguana, Giant (MC5: Monstrous Compendium Greyhawk Appendix) [2e]
Iguana (Dragon #341) [3.5]
Ihagnim (Dragon #89) [1e]
Imago (Savage Species Web Enhancement) [3.0]
Imbul (Practical Planetology (SJR4)) [2e]
Imorph (Fiend Folio) [1e]
Imp, Blue (GAZ3: Principalities of Glantri) [BECMI]
Imp, Bog (Mystara Monstrous Compendium Appendix) [2e]
Imp, Book (Arcane Power) [4e]
Imp, Chaos (Planes of Chaos/Monstrous Compendium Annual Volume Three) [2e]
Imp, Choleric (Dragon #338) [3.5]
Imp, Garden (Mystara Monstrous Compendium Appendix) [2e]
Imp, Melancholic (Dragon #338) [3.5]
Imp, Phlegmatic (Dragon #338) [3.5]
Imp, Red (GAZ3: Principalities of Glantri) [BECMI]
Imp, Sanguine (Dragon #338) [3.5]
Imp, Wishing (Ravenloft Monstrous Compendium Appendix III: Creatures of Darkness) [2e]
Incarnate (Dragon #193/Planescape Monstrous Compendium Appendix) [2e]
Ineffable Horror (Underdark) [3.5]
Inevitable, Anhydrut (Waste Crawler) (Sandstorm) [3.5]
Inevitable, Kolyarut (Manual of the Planes/Monster Manual v.3.5) [3.5]
Inevitable, Quarut (Fiend Folio ) [3.0]
Inevitable, Varakhut (Fiend Folio ) [3.0]
Inevitable, Zelekhut (Manual of the Planes/Monster Manual v.3.5) [3.0]
Infernal Armor (D&D Miniatures: Demonweb) [4e]
Infernite (Dragon #159) [2e]
Infinity Vine (MC7: Monstrous Compendium Spelljammer Appendix) [2e]
Ingogo (Dragon #122) [1e]
Insectoid, Antold (Dragon #32) [1e]
Insectoid, Koasp (Dragon #32) [1e]
Insectoid, Scorpiorc (Dragon #32) [1e]
Insectoid, Skag (Dragon #32) [1e]
Insectoid, Woblin (Dragon #32) [1e]
Irda Tree (Sylvan Veil) [2e]
Iridescent Ooze (Dragon #367) [4e]
Iridescent Plecoe (Ruins of Undermountain II/Monstrous Compendium Annual Volume Two) [2e]
Iron Maiden (Polyhedron #145) [3.0]
Iron Man (C3: Lost Island of Castanamir) [1e]
Ironclad Mauler (Monster Manual III) [3.5]
Ironglass Rose (Wizards of the Coast web site) [3.5]
Ironthorn (Sandstorm) [3.5]
Island Serpent (OA3: Ochimo the Spirit Warrior) [1e]
Isopterite (SJA3: Crystal Spheres) [2e]
Ivory Champion (Dragon #356) [3.5]