GURPS Gulliver is a rules expansion for GURPS 3rd Edition Revised covering the creation and scaling of creatures under that system. From the website's introduction:
"GULLIVER focuses on all things physical for characters: building and gaming life forms of any shape or size. It's the biggest expansion ever on that topic for GURPS (or maybe any RPG). Picture a GURPS Vehicles for creatures, with a good dose of biomechanical science as its base.
"It is not a single use-it-or-don't-use-it item. Not at all. It's a collection of independent and semi-independent rules additions. Many add detail and complexity. Some simplify GURPS and make it easier to play. Together, they create a more universal, generic rules set that works equally under extreme scales/situations and under normal human conditions. But you can pick as many or as few parts as you like, and mix and match as you please."
"Topics in GULLIVER include, but are not limited to, the following: Size, scaling and stats; shrinking and growing; ST and HT and HP and Fatigue and making them get along; natural encumbrance; weight vs mass; encumbrance on land, in water, in air, in space; powered vs unpowered flight; density and buoyancy; disadvantages and environment; gas bags and swim bladders and static lift; odd shapes and TH; fat and energy stores; bioluminescence and weird venom enhancements; all about thermal regulation; solution for "DR cheaper than HP"; new weaknesses and tolerances; skeletons, exoskeletons and invertebrates; movement and drag; every type of flight imaginable; airfoils, load ratings and other advanced wing design; grip and manual dexterity; new strikers; multiple brains; slime; antennae; deer and headlights; movement and resisting weight and fatigue; acceleration and deceleration; crawling and flopping; aquatic jumping; running takeoffs and too-fast landings; air brakes; microgravity movement; weight, and climbing; non-muscle jumping; jump angle, body position and distance; jumping speed, acceleration, air time, air resistance and escape velocity; one-handed, two-handed, and four-armed throws; throwing speed, angle, and running throws; baseball pitching and other sports throws; balance and off-balance; detailed Stealth modifiers; generic Extra Effort; target size, weapon size, Dodge, and TH; speed and range in melee combat; parrying massive attacks; size and shields and big attacks; size and weight and wrestling; pushes and slams; bear hugs and crushing; rending and amputation; weight and damage; physics-friendly judo throws; how to be saved by knockback; dealing with high AD; size and shock and damage; handling split HP; new collision and falling systems; size, poison and disease; cube-square effects and exposure hazards; light and dark; size, food requirements, and expected life span; make-your-own appendage builder system; life span meta system; scaling big and small games; size and supernormal abilities; tiny games; big games; bunny games; shrinking rays and enlargement spells; Somakinesis; underwater games; sample fantasy campaign background; sample aquatic campaign background."