Mysterious Fossil occupies a Pokémon's spot in play as a Basic Pokémon with no attacks that can't retreat or be affected by Special Conditions, and can be discarded from play during its owner's turn before attacking. Legendary Collection's wording added that Pokémon evolving from it can be placed directly on the card, and Legend Maker's wording reversed the earlier rule so it counts as a Trainer card as well as a Pokémon, rather than instead of one, and being Knocked Out still doesn't count as a Knocked Out Pokémon.
Mysterious Fossil first appeared in Fossil in 1999 with art by Keiji Kinebuchi, who also illustrated the Legendary Collection and Sandstorm reprints. Ryo Ueda took over for the Legend Maker printing in 2006 and illustrated its subsequent reprints in Holon Phantoms and Power Keepers, carrying the card through the EX era. All six of its printings are Common rarity.
Fossils are items in the Pokémon games that can be revived into Pokémon. The Dome Fossil and Helix Fossil appear in this card's artwork, though the illustration depicts the Dome Fossil as the fossil of a Kabutops rather than a Kabuto, alongside some wing bones from Aerodactyl, a small but notable inconsistency in the card's design.
Fossil printing is Mysterious Fossil's first appearance, illustrated by Keiji Kinebuchi
Legendary Collection printing adds the clause allowing evolution cards to be placed on it
Legend Maker printing reverses the Trainer card counting rule and introduces Ryo Ueda's artwork
Its wording reversed so the card counts as a Trainer card as well as a Pokémon, rather than instead of one.
The Dome Fossil and Helix Fossil, along with some Aerodactyl wing bones, though the Dome Fossil is drawn as a Kabutops fossil rather than Kabuto.
Ryo Ueda illustrated the Legend Maker, Holon Phantoms, and Power Keepers printings.
From Fossil (1999) · 1 printing
Play Mysterious Fossil as if it were a Basic Pokémon. While in play, Mysterious Fossil counts as a Pokémon (instead of a Trainer card). Mysterious Fossil has no attacks, can't retreat, and can't be Asleep, Confused, Paralyzed, or Poisoned. If Mysterious Fossil is Knocked Out, it doesn't count as a Knocked Out Pokémon. (Discard it anyway.) At any time during your turn before your attack, you may discard Mysterious Fossil from play.
From Legendary Collection (2002) · 1 printing
Play Mysterious Fossil as if it were a Basic Pokémon. While in play, Mysterious Fossil counts as a Pokémon (instead of a Trainer card). Mysterious Fossil has no attacks, can't retreat, and can't be affected by Special Conditions. If Mysterious Fossil is Knocked Out, it doesn't count as a Knocked Out Pokémon. (Discard it anyway.) At any time during your turn before your attack, you may discard Mysterious Fossil from play. You can put Pokémon that evolve from Mysterious Fossil on this card.
From Sandstorm (2003) · 1 printing
Play Mysterious Fossil as if it were a Basic Pokémon. While in play, Mysterious Fossil counts as a Colorless Pokémon (instead of a Trainer card). Mysterious Fossil has no attacks of its own, can't retreat, and can't be affected by any Special Conditions. If Mysterious is Knocked Out, it doesn't count as a Knocked Out Pokémon. (Discard it anyway.) At any time during your turn before your attack, you may discard Mysterious Fossil from play.
From Legend Maker (2006) · 3 printings
Play Mysterious Fossil as if it were a Basic Pokémon. While in play, Mysterious Fossil counts as a Colorless Pokémon (as well as a Trainer card). Mysterious Fossil has no attacks of its own, can't retreat, and can't be affected by any Special Conditions. If Mysterious Fossil is Knocked Out, it doesn't count as a Knocked Out Pokémon. (Discard it anyway.) At any time during your turn before your attack, you may discard Mysterious Fossil from play.

Mysterious Fossil
Item
First printed in Fossil (1999)
Play Mysterious Fossil as if it were a Basic Pokémon. While in play, Mysterious Fossil counts as a Colorless Pokémon (as well as a Trainer card). Mysterious Fossil has no attacks of its own, can't retreat, and can't be affected by any Special Conditions. If Mysterious Fossil is Knocked Out, it doesn't count as a Knocked Out Pokémon. (Discard it anyway.) At any time during your turn before your attack, you may discard Mysterious Fossil from play.