Let’s look at the ACKS monster list and see what fits here:
Ankheg
Ant, Giant
Basilisk
Bee, Giant Killer
Beetle, Giant
Black Pudding
Caecilian
Caracass Scavenger
Centipede, Giant
Crab, Giant
Crocodile
Doppleganger (note sure it’ll fit well)
Dragon Turtle
Elemental
Fish, Giant
Fly, Giant Carnivorous
Gelatinous Cube
Ghoul
Golem (especially Flesh)
Gray Ooze
Green Slime
Herd Animals (representing small to medium dinosaurs)
Invisible Stalker
Leech, Giant
Lizardman
Lizard, Giant
Locust, Cavern
Men
Morlock
Mummy (usually of a Lizardman)
Ochre Jelly
Octopus, Giant
Pterodactyl (needs house-rules for riding)
Purple Worm
Remorhaz
Rhagodessa, Giant
Rot Grub
Rust Monster (maybe?)
Scorpion, Giant
Sea Serpent
Shadow
Shark
Shrieker
Skeleton
Skittering Maw
Snake
Spectre (usually representing powerful spirits summoned by Lizardman Shamans)
Spider, Giant
Squid, Giant
Statue, Animated
Stegosaurus (might need a slight deflation of HD)
Stirge
Swarm (Insect or small reptile - maybe even a snake swarm?)
Toad, Giant
Triceratops (possibly trainable as a massive draft animal)
Tyrannosaurus Rex (needs a significant HD deflation, way too many HD for an animal of it size; most whales have less HD)
Wraith (another spirit for Lizardman Shamans to summon)
Yellow Mold
Zombie
Let’s look at the D20 SRD (http://www.d20srd.org/) monster list and see what fits here:
Aboleth and Aboleth Mage
Allip (another spirit for Lizardman Shamans to summon)
Aranea
Assassin Vine
Behir
Chaos Beast (isn’t that a Shoggoth?)
Choker
Chuul
Cloaker
Couatl (now THAT’s what high-level Lizardman Shamans summon)
Darkmantle
Destrachan
Digester
Dinosaur, Deinonychus
Dinosaur, Elasmosaurus
Dinosaur, Megaraptor
Ethereal Filcher
Ethereal Marauder
Ettercap
Formian
Frost Worm
Fungus, Violet Fungus
Gibbering Mouther (another Shoggoth candidate!)
Grick
Homunculus (needs house-rules for creation)
Kraken
Mimic (actually a cursed chest)
Mohrg
Mummy Lord
Naga
Otyugh
Phantom Fungus
Phase Spider
Phasm (Shoggoth?)
Roper
Shadow, Greater
Shambling Mound
Shield Guardian (reskinned into Security Robot)
Shocker Lizard
Skum
Spider Eater
Tendriculos
(i)Thoqqua
Yrthak
Lizard, Monitor
Monsters that need adding as house-rules:
Alamosaurus
Ankylosaurus
Avatar of K’tulu
Cacodemon
Carnotaurus (possibly ridable)
Charonosaurus
Dagon
Dark Young
Deep Ones
Edmontosaurus (probably ridable)
Frog, Giant
Lizard, Komodo Dragon
Lizard, Megalania
Lizard, Mosasaurus
Lizard Brute (think of a a dim-witted Lizardman the size of an ogre or hill-giant)
Lizard Brute, Greater (regenerates like a Troll)
Magyarosaurus (possibly ridable)
Plant, Carnivorous
Plesiosaurs
Pyroraptor (maybe a replacement for rats?)
Serpentman, Hybrid
Serpentman, Degenerate
Serpentman, Undead Sorceror
Servitour of K’Tulu
Shoggoth
Toadmen
Torosaurus (possibly trainable as a draft animal and/or raised for meat and eggs)
Troodon (SMART! maybe a common pet and/or guard animal)
Wasp, Giant, Parasitic