From Bulbapedia, the community-driven Pokémon encyclopedia.
Ancient Traits are special effects given to certain Pokémon, starting in the Primal Clash expansion. They are not Abilities, and as such, effects that would prevent Abilities from activating (such as Garbodor's Garbotoxin or Wobbuffet's Bide Barricade) do not apply.
All Ancient Trait cards except for Pokémon-EX with Ancient Trait have artwork that differs from normal cards. The artwork takes up the top 3/4 of the card; it then fades into the bottom 1/4, which is colored depending on the Pokémon's type. For example, an Entei card would have its bottom 1/4 colored red. They are commonly referred to as "half art cards" for this reason.
The Ancient Trait bar and text is placed at the top of the card, beneath the Pokémon's name. Because the artwork takes up most of the space available, the basic information of the Pokémon (Pokédex number, species, height and weight) is not shown.
Ancient Trait cards are treated as regular cards in play; they can be evolved from and into regular cards along its evolutionary line. A Pokémon's Ancient Trait does not depend on its type or rarity, though certain Ancient Traits are unique to the Weather Trio (∆ Wild and θ Max).
Whenever an Ancient Trait card appears in a TCG set, a regular card of the same Pokémon appears as well (except for Pokémon-EX with Ancient Trait). Ancient Trait cards are usually stronger than their regular counterparts. Many of their attacks also take advantage of its Ancient Trait. Examples include α Growth Kingdra and Primal Kyogre-EX (which discard two energy each turn) or α Recovery Eelektrik and Azumarill (which causes recoil damage and attacks more effectively with more HP).
List of Ancient Traits
Description: When this Pokémon is healed, double the amount healed.
Effect: When the Pokémon recovers HP, the amount recovered is doubled.
Description: When you attach an Energy card from your hand to this Pokémon (except with an attack, Ability, or Trainer card), you may attach 2 Energy cards.
Effect: When an Energy card is attached to the Pokémon from the player's hand without using an Attack, Ability or Trainer card, a second Energy card may be attached if the player has one in their hand.
Description: This Pokémon may attack twice a turn. (If the first attack Knocks Out your opponent's Active Pokémon, you may attack again after your opponent chooses a new Active Pokémon.)
Effect: The Pokémon may attack twice in a turn. If the first attack Knocks Out the opponent's Active Pokémon, the second attack hits the new one.
Description: Whenever your opponent plays a Trainer card (excluding Pokémon Tools and Stadium cards), prevent all effects of that card done to this Pokémon.
Effect: The effects of the opponent's Trainer cards (other than Pokémon Tools and Stadium cards) do not apply to the Pokémon.
Description: If your opponent's Pokémon is Knocked Out by damage from an attack of this Pokémon, take 1 more Prize card.
Effect: When the Pokémon Knocks Out an opposing Pokémon, the player takes one extra Prize card.
Description: You may play this card from your hand to evolve a Pokémon during your first turn or the turn you play that Pokémon.
Effect: The Pokémon may be played to evolve a Pokémon during the same turn it was put into play, or on the player's first turn.
Description: Any damage done to this Pokémon by attacks from your opponent's , , , or Pokémon is reduced by 20 (after applying Weakness and Resistance).
Effect: The Pokémon takes 20 less damage from , , , and Pokémon.
Description: This Pokémon cannot be affected by your opponent's Pokémon's Abilities.
Effect: The Pokémon is unaffected by opposing Pokémon's Abilities.
Description: This Pokémon can have 2 Pokémon Tool cards attached to it.
Effect: An extra Pokémon Tool can be attached to the Pokémon.
Description: When 1 of your Pokémon becomes this Pokémon, heal all damage from it.
Effect: When the Pokémon evolves from a Pokémon in play, all damage counters are removed from it.