Special Abilites
Each version of a polymorph spell has a maximum CR. When you design a polymorph form, the target can gain the same maximum number of special abilities as an expert NPC of a CR equal to the spell’s maximum CR. Alternatively, if you base your form on a specific creature (which must be of a CR no higher than the spell’s maximum CR), you can select only abilities that the base form has, but you can select one ability more than the normal maximum.
All of these are in addition to any abilities gained from the polymorph form’s type or subtype (which are limited—see Type and Subtype), and they must be selected from the following list: amphibious, breath weapon, breathing, compression, defensive abilities, limited telepathy, racial traits, senses, and trample.
Breath Weapon: A breath weapon deals damage of one energy type of your choice, deals 1d4 damage of that type per character level or CR of the polymorphed target (to a maximum equal to the polymorph effect’s maximum CR), must be a line or cone (following the guidelines for the breath weapon universal creature rule), can be used only once every 4 rounds, and does one less d4 of damage each time it is used. When the damage reaches 0d4, the breath weapon no longer functions. A target can attempt a Reflex save (DC = 10 + half the polymorphed creature’s character level or CR or 10 + half the maximum CR of the polymorph effect, whichever is lower) to reduce the damage by half.
Breathing: When selecting breathing as a special ability, you can select one environment in which you are aware that living creatures can natively exist, and grant your polymorph form the ability to breathe in that specific environment. This can be underwater, in a specific toxic environment, or even in the void of space if you have first-hand knowledge of living creatures that successfully operate in a vacuum without needing to breathe. The target retains the ability to breathe as it normally does in addition to this benefit. Breathing can be an unimportant feature if the target retains its armor’s environmental protections (see Equipment).
Defensive Abilities: The limits of which defensive abilities you can select are outlined in each polymorph spell. You can select only one of the defensive options listed for the level at which you cast the spell, and it counts toward the maximum number of special abilities the form can grant.
Movement: Each polymorph spell or ability details what changes to movement, if any, your polymorph form can grant. Your polymorph form can grant one form of movement without it counting against its maximum number of special abilities, but any additional forms of movement must be selected as special abilities.
Racial Traits: When selecting a racial trait for a polymorph form, you can select any one player character racial trait of a race that grants such rules and that does not refer to or require any equipment, armor, armor upgrade, or drone upgrade to function. For example, you could select the exceptional vision trait of an android (gaining both low-light vision and darkvision), but not the upgrade slot ability (since it refers to armor upgrades). Additionally, the target can never gain racial ability adjustments, racial Hit Points, feats, or skill ranks from a polymorph spell’s effects. Each level of the polymorph spell has specific restrictions regarding spell-like abilities.
Senses: The limits of what senses you can select are outlined in each polymorph spell. Each of the sense options a given polymorph spell can grant the target counts as a single special ability that counts toward the maximum number of special abilities a polymorph form can grant.
Other Abilities: If you’re designing a polymorph form based on a specific creature, the GM may allow you to select one of that creature’s special abilities or traits so long as the creature has a CR no higher than the spell’s maximum CR. If a polymorph spell grants a trait that uses a base creature’s CR for a calculation, instead use the spell’s maximum CR or the polymorphed creature’s character level or CR, whichever is lower.
Some abilities should not be permitted by a GM. The target can’t gain an aura from a polymorph form. An ability can’t be gained if it requires more time to use than the polymorph spell’s duration. The target can’t gain any ability that makes it part of a communal mind or body, such as the barathu combine ability or the swarm mind ability that swarms have. In addition, the target can’t gain any special ability that allows it to summon or otherwise spawn other creatures. Because of how polymorph effects interact (per the Multiple Effects section), a polymorph spell can’t grant the target the change shape special ability, since the polymorph spell that granted the ability would end when the target used it. If an ability is supernatural or could harm the target in its use, the GM should be very cautious before allowing access to it in a polymorph form.
The weird magic of polymorph spells might grant the target the ability to cause other effects that should last longer than its transformation. If an effect the target creates while in a polymorph form has an ongoing duration when the target returns to normal form, the effect ends unless the GM decides otherwise. However, changes the target made that are not ongoing effects, such as the death of enemies the target fought, are permanent.
Source
Alien Archive 2 pg. 142