Megalania

Megalania (Varanus Priscus to the taxonomists) is a Pleistocene ancestor of the modern monitor lizard, growing to lengths that make it a terrifying apex predator and an ideal heavy combat mount for Medium reptilian riders like the khaasta. They are low-slung, heavily armored with dense scales and possess a sweeping tail. Some species have a highly toxic bite.

Combat

A megalania typically charges into melee, using its immense weight to pin a target down with improved grab while its venom rapidly weakens the prey.

Improved Grab (Ex): To use this ability, a megalania must hit with its bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and delivers automatic bite damage each round until the target escapes the grapple or dies. If the megalania is venomous, it inflicts that venom each round of the grapple.

Venom (Ex): Some megalania specimens have one of two venom variants depending on their regional breed. The save DC is Constitution-based.

• Anticoagulant (bleed strain): Injury, Fortitude DC 19, initial and secondary damage 1 point of persistent bleeding damage per round. Multiple applications stack. The bleeding can be stopped with a DC 15 Heal check or any magical healing spell.
• Neurotoxin (paralysis strain): Injury, Fortitude DC 19, initial damage 1d4 Dex, secondary damage paralysis for 1d4 rounds.

Skills: A megalania has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It uses its Strength modifier or Dexterity modifier for Swim checks, whichever is higher.

A megalania has a +4 racial bonus on Hide and Move Silently checks. In forested or overgrown areas, the Hide bonus improves to +8

Source: Archive Three-Five. These were inspired by the khaasta entry in Fiend Folio describing but not detailing giant lizard mounts. In D&D 3.0, the giant lizard was what became the monitor lizard in the Monster Manual (version 3.0), but both of those are Medium⁠—​too small to be used as a mount for a Medium size creature. Some study on Wikipedia into the monitor lizard revealed the megalania, an extinct Pleistocene ancestor that was of suitable size. This creature is based on the statistics block on the monitor lizard in the Standard Reference Document (SRD) under Open Game License v1.0a, with increases in hit dice to make it bigger, and an increase in size category to make it suitable for a mount for a Medium size creature. The size of the skeletons of some the megalania discovered justifies the Advancement entry. Additional features were added based on non-extinct species of monitor lizards, including scent and venom.