Certain creatures are created by adding a template to an existing creature. A templated creature can represent a freak of nature, the individual creation of a single experimenter, or the first generation of offspring from parents of different species.
Some templates can be added to creatures anytime. Templates such as these are referred to as acquired templates, indicating that the creature did not always have the attributes of the template.
Other templates, known as inherited templates, are part of a creature from the beginning of its existence. Creatures are born with these templates.
It's possible for a certain kind of template to be of either type.
A template's description provides a set of instructions for altering an existing creature, known as the base creature. The changes that a template might cause to each line of a creature's statistics block are discussed below. Generally, if a template does not cause a change to a certain statistic, that entry is missing from the template description. For clarity, the entry for a statistic or attribute that is not changed is sometimes given as 'Same as the base creature.'
Size and Type: Templates often change a creature's type, and may change the creature's size.
If a template changes the base creature's type, the creature also acquires the augmented subtype unless the template description indicates otherwise. The augmented subtype is always paired with the creature's original type. Unless a template indicates otherwise, the new creature has the traits of the new type but the features of the original type.
If a template changes a creature's size, use Table: Changes to Statistics by Size to calculate changes to natural armor, Armor Class, attack rolls, and grapple bonus.
Hit Dice and Hit Points: Most templates do not change the number of Hit Dice a monster has, but some do. Some templates change the size of a creature's Hit Dice (usually by changing the creature type). A few templates change previously acquired Hit Dice, and continue to change Hit Dice gained with class levels, but most templates that change Hit Dice change only the creature's original HD and leave class Hit Dice unchanged.
If the Hit Dice entry in a template description is missing, Hit Dice and hit points do not change unless the creature's Constitution modifier changes.
Initiative: If a template changes the monster's Dexterity, or if it adds or removes the Improved Initiative feat, this entry changes.
Speed: If a template modifies a creature's speed, the template states how that happens. More commonly, a template adds a new movement mode.
Armor Class: If a template changes the creature's size, see Table: Changes to Statistics by Size to determine its new Armor Class and to see whether its natural armor changes. In some cases the method of determining Armor Class changes radically; the template description explains how to adjust the creature's AC.
Base Attack/Grapple: Templates usually do not change a creature's base attack bonus. If a template modifies a creature's base attack bonus, the template description states how that happens. Changes to a creature's Strength score can change a creature's grapple bonus, as can changes to its size.
Attack and Full Attack: Most templates do not change a creature's attack bonus or modes of attack, even when the creature's type changes (the creature's base attack bonus is the same as a creature of the original type). Of course, any change in ability scores may affect attack bonuses. If Strength or Dexterity changes, use the new modifier to determine attack bonuses. A change in a monster's size also changes its attack bonus; see Table: Changes to Statistics by Size.
Damage: Damage changes with Strength. If the creature uses a two-handed weapon or has a single natural weapon, it adds 1-1/2 times its Strength bonus to the damage. If it has more than a single attack then it adds its Strength bonus to damage rolls for the primary attack and 1/2 its Strength bonus to all secondary attacks.
Space/Reach: A template may change this entry if it changes the monster's size. Note that this table does not take into account special situations such as exceptional reach.
Special Attacks: A template may add or remove special attacks. The template description gives the details of any special attacks a template provides, including how to determine saving throw DCs, if applicable.
Special Qualities: A template may add or remove special qualities. The template description gives the details of any special qualities a template provides, including how to determine saving throw DCs, if applicable. Even if the special qualities entry is missing from a template description, the creature still gains any qualities associated with its new type.
Base Saves: As with attacks, changing a monster's type does not always change its base saving throw bonuses. You only need to adjust them for new modifiers for Constitution, Dexterity, or Wisdom. A template may, however, state that a monster has a different 'good' saving throw.
Skills: As with attacks, changing a monster's type does not always change its skill points. Most templates don't change the number of Hit Dice a creature has, so you don't need to adjust skills in that case unless the key abilities for those skills have changed, or the template gives a bonus on one or more skills, or unless the template gives a feat that provides a bonus on a skill check.
Some templates change how skill points are determined, but this change usually only affects skill points gained after the template is applied. Treat skills listed in the base creature's description as class skills, as well as any new skills provided by the template.
Feats: Since most templates do not change the number of Hit Dice a creature has, a template will not change the number of feats the creature has. Some templates grant one or more bonus feats.
Challenge Rating: Most templates increase the creature's Challenge Rating. A template might provide a modifier to be added to the base creature's CR, or it might specify a range of modifiers depending on the base creature's original Hit Dice or CR.
Alignment: Usually the same as the base creature, unless the template is associated with a certain alignment.
Level Adjustment: This entry is a modifier to the base creature's level adjustment. Any level adjustment is meaningless unless the creature retains a high enough Intelligence (minimum 3) to gain class levels after applying the template.
In theory, there's no limit to the number of templates you can add to a creature. To add more than one template, just apply each template one at a time. Always apply inherited templates before applying acquired templates. Whenever you add multiple templates, pay attention to the creature's type-you may add a template that makes the creature ineligible for other templates you might want to add.