(hex:A8) is a glitch item in Pokémon Yellow. It is an unterminated name glitch item and a Rival's effect glitch item.
When a 0x50 sub-tile is early in the saved screen data, this glitch item apparently has no effect other than the Rival's effect — if it is attempted to be used on a Pokémon the message that the item won't have any effect will appear.
Without the 0x50 sub-tile, pressing A on this glitch item may cause the game to freeze.