Fossil is a set in the Base series. It was released on October 10, 1999. The printed size is 62, and the checklist is the same length. Ditto is 3/62, in position 3 of 62 on this checklist. Before it is 2 Articuno, and after it is 4 Dragonite. The listed rarity is Rare Holo.
Ditto is a Basic Colorless Pokémon with 50 HP. The card lists level 20. In the Pokédex this is Ditto (#132), generation 1 (Kanto).
Keiji Kinebuchi illustrated this card. The same name is on 212 other cards in the catalog.
Ditto (#132) has 31 other cards in the catalog. Those other cards are in 20 sets. The other printings with a release date run from 1999 to 2026. The earliest other card is Ditto 18/62 in Fossil (1999). Both list 50 HP. The rarity there is Rare, and here it is Rare Holo. 29 other cards of Ditto list a different HP.
Transform is listed as Pokémon Power. If Ditto is your Active Pokémon, treat it as if it were the same card as the Defending Pokémon, including type, Hit Points, Weakness, and so on, except Ditto can't evolve, always has this Pokémon Power, and you may treat any Energy attached to Ditto as Energy of any type. Ditto isn't a copy of any other Pokémon while Ditto is Asleep, Confused, or Paralyzed. Weakness is Fighting ×2. Resistance is Psychic -30. Retreat cost is 1. The flavor text reads "Capable of copying an enemy's genetic code to instantly transform itself into a duplicate of the enemy."
I keep a set binder for Fossil, meant to hold every card of that set. This card is one slot in the Fossil set binder. A generation binder holds one card of each species. Ditto can fill a slot in Generation 1 binder (Kanto). I still need this card.
AbilityTransform
Pokémon Power
If Ditto is your Active Pokémon, treat it as if it were the same card as the Defending Pokémon, including type, Hit Points, Weakness, and so on, except Ditto can't evolve, always has this Pokémon Power, and you may treat any Energy attached to Ditto as Energy of any type. Ditto isn't a copy of any other Pokémon while Ditto is Asleep, Confused, or Paralyzed.
Flavor text
Capable of copying an enemy's genetic code to instantly transform itself into a duplicate of the enemy.