Size and Traits
Source GM Core pg. 117
Fill out the trait line of your creature's stat block. Creatures can be whatever size you need them to be, though you seldom find Large creatures below 1st level, Huge creatures below 5th level, or Gargantuan creatures below 10th level. Generally, you don't automatically adjust statistics for size, except to Strength modifiers for Large and bigger creatures, which you'll find in the following Attribute Modifiers section.
Your creature will almost certainly have one of the following traits to define its type: aberration, animal, astral, beast, celestial, construct, dragon, elemental, ethereal, fey, fiend, fungus, giant, humanoid, monitor, ooze, plant, robot, or undead. If you're making a creature from an existing category of a type, such as demon, it also has that category as a trait. Creatures with a close affinity to elements—air, earth, fire, metal, water and wood—or types of energy—like acid, cold, and electricity—have those traits.
Some abilities that are typical of creatures with the traits listed here can be found in Trait Abilities. As with the other steps, looking at similar creatures will give you an idea of what traits to use.
Add any traits that have detailed rules attached to them, like amphibious, aquatic, cosmic, incorporeal, mindless, and swarm. You can add traits related to the creature category, such as kucharn or midwife, but most of these traits are pretty self-evident in play. If at any point you realize during play that you didn't add a trait the creature really should have, you can usually apply it retroactively.
Your creature will almost certainly have one of the following traits to define its type: aberration, animal, astral, beast, celestial, construct, dragon, elemental, ethereal, fey, fiend, fungus, giant, humanoid, monitor, ooze, plant, robot, or undead. If you're making a creature from an existing category of a type, such as demon, it also has that category as a trait. Creatures with a close affinity to elements—air, earth, fire, metal, water and wood—or types of energy—like acid, cold, and electricity—have those traits.
Some abilities that are typical of creatures with the traits listed here can be found in Trait Abilities. As with the other steps, looking at similar creatures will give you an idea of what traits to use.
Add any traits that have detailed rules attached to them, like amphibious, aquatic, cosmic, incorporeal, mindless, and swarm. You can add traits related to the creature category, such as kucharn or midwife, but most of these traits are pretty self-evident in play. If at any point you realize during play that you didn't add a trait the creature really should have, you can usually apply it retroactively.