Whatever megafauna you intend to create and whatever method you choose to utilize, megafauna have numerous similarities and key features, including the following statistics.

Alignment: Megafauna are neutral in alignment.

Intelligence: Megafauna have Intelligence scores of –4 or –5.

Senses: Most megafauna have either low-light vision or darkvision. Other common senses megafauna might have include echolocation (as a precise sense) and scent or tremorsense (as imprecise senses).

Special Defenses: Although not impossible, it's rare for megafauna to have special defenses such as regeneration, immunities, resistances, or weaknesses. Include them only if they are key to your concept.

Skills: Megafauna have only a few skills, which are focused on their physical abilities and methods of survival. These are usually Acrobatics, Athletics, Stealth, or Survival.
Traits: Megafauna have the animal trait. Dinosaurs have the dinosaur trait, and creatures that live in the water might have the aquatic or amphibious traits.

Omissions: Megafauna don't have languages, items, or spells. They almost never have magical abilities.