Mysterious Treasures is a set in the Diamond & Pearl series. It was released on August 1, 2007. The printed size is 123. This checklist is longer, at 124 cards. Unown [E] is 65/123, in position 65 of 124 on this checklist. Before it is 64 Tropius, and after it is 66 Unown [M]. The listed rarity is Uncommon.
Unown [E] is a Basic Psychic Pokémon with 50 HP. The card lists level 18. In the Pokédex this is Unown (#201), generation 2 (Johto).
Daisuke Ito illustrated this card. The same name is on 47 other cards in the catalog.
Unown (#201) has 95 other cards in the catalog. Those other cards are in 18 sets. The other printings with a release date run from 1999 to 2026. The earliest other card is Unown [J] 38/53 in Wizards Black Star Promos (1999). That card is named Unown [J], while this one is Unown [E]. That card lists 40 HP, and this one lists 50 HP. The rarity there is Promo, and here it is Uncommon. Hideki Kazama illustrated that card, and Daisuke Ito illustrated this one. 67 other cards of Unown list a different HP.
EQUIP is listed as Poké-Power. Once during your turn (before your attack), if Unown E is on your Bench, you may discard all cards attached to Unown E and attach Unown E to 1 of your Pokémon as a Pokémon Tool card. As long as Unown E is attached to a Pokémon, that Pokémon gets +10 HP. The attack Hidden Power lists no damage number. The cost is Colorless, Colorless. During your opponent's next turn, whenever your opponent flips a coin, treat it as tails. Weakness is Psychic +10. Retreat cost is 1. The flavor text reads "Shaped like ancient writing, it is a huge mystery whether language or UNOWN came first."
I keep a set binder for Mysterious Treasures, meant to hold every card of that set. This card is one slot in the Mysterious Treasures set binder. A generation binder holds one card of each species. Unown can fill a slot in Generation 2 binder (Johto). I still need this card.