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.