A few more:
Aboleth: Looks like a giant jellyfish. Floats in air (not water), with tendrils hanging down.
Allip: “Tortured Flame” – created from tortured fire elementals and salamanders.
Archon – Lantern: "Storm Cloud" – same stats, throws lightning bolts, loud thunder in battle.
Azer: “Burning Orcs”. Same stats, but use orcs as base creature.
Basilisk: "Loch Dweller" – same stats, long serpentine body, lives in water. Swim speed and hold breath. Tactics include popping up out of the water, stoning its victims, then diving back under the water to avoid attacks. Rinse and repeat. CR should probably increase by 1 or 2.
Beholder: A floating ball of multicolored light. Eye rays are beams of light it emits. Instead of bite substitute energy drain (it draws the life force from its victims, leaving a desiccated husk behind). Alternately, use prismatic spray for eye beams.
Centaur: A race of half-man, half-beetle that dwell underground. Darkvision and spider climb as Ex abilities.
Choker: Centipede with two tentacles. Spider climb as Ex ability.
Cockatrice: "Stone Crawler" – looks like long, multi-legged vermin. Can spider climb on walls and ceilings. Consumes “living stone”, which is created by stoning a living creature.
Destrachan: "Bane Bat" – same stats, flight.
Displacer Beast: "Bridge Troll" – same stats, tentacles become arms which can stretch out (rubbery). If add regeneration, up CR by 1.
Duergar: "Changeling" – same stats, human stolen as baby and raised as elf.
Gargoyle: “Stone Hound”, same stats, very large canine with wings.
Girallon: Ogre with four arms.
Girallon, Fiendish: "Battle Demon". Fiendish adds DR 5/magic, SR 12, cold and fire resistance 5. CR +1.
Girallon: "Green Martian" (loosely inspired by John Carter of Mars series). Tall, thin, four arms, green-skinned humanoid. Jagged bones which jut from the wrist and back of the hand tear flesh. Increase average Int to 10. Desert dweller.
Girallon: Giant centipede. In combat, part of its body rears up, allowing it to attack with four feat/claws.
Gorgon: Put in minotaur body, with gorgon head.
Krenshar: Hooded humanoid undead. If hood is pulled back, reveals a maggot-ridden skull. Keening substitutes for krenshar’s screech.
Halfling: "Hel-ling". Fiendish halflings with small bat wings.
Howler: "Spine lizards". Shoots quills from its tail. Wisdom drain caused by a deep-throated thrumming.
Lillend: “Foamfollower” – same stats, swim speed instead of flight (no wings). Relative of merfolk?
Roper: Living web.
Satyr: "Sea Singer" – same stats, add swim speed. Merfolk-like creature.
Shadow: Energy being from another plane. Travels to prime material to spawn. Substitute chameleon ability (+4 to all Hide checks) for shadow's bonus to Hide in dark areas. Outsider, not undead.
Sprite: Intelligent insect, wasp-like. Instead of arrows, it stings its victims, injecting its choice of poison (memory loss, sleep). Casts spells by changing the drone of its wings.
Stirge: Vampire bat.
Winter Wolf: Cerberus, same stats, 3-headed hell hound. Still gets only one bite attack. Substitute fire damage for cold damage.