From Bulbapedia, the community-driven Pokémon encyclopedia.
Who's That Pokémon (Japanese: Dare Da?) was part of all Pokémon anime episodes from EP001 to AG040 in the dub and EP001 to EP274 in Japan. It occurred before the main commercial during the episode. A silhouette of a Pokémon is shown and the viewer guesses its identity. The name of the Pokémon is revealed after the commercial break.
Throughout the Japanese version, and from early to mid-Kanto in the dub, the Who's That Pokémon almost always featured a Pokémon that played a major role in that episode. Beginning in late Kanto, the Who's That Pokémon of the dub was usually one that was featured in the following episode. When Johto League Champions began, Who's That Pokémon became random, with no way to predict who the Pokémon would be. Also starting with Johto League Champions, each segment showed a hint to help identify the pokemon.
In the dub Who's that Pokémon was eliminated in Pokémon Advanced Challenge and replaced with Pokémon Trainer's Choice. In the Japanese version, it ended at the end of Pocket Monsters and was replaced in the Advanced Generation series with a generic eyecatch showing the main characters, which continued into the Diamond & Pearl series.
In the Japanese version of Beauty and the Beach, the Who's that Pokémon was Brutella. In the Who's That Pokémon for The Bug Stops Here Jessie was dressed as Venomoth in the Japanese version. When the "Pokémon" was revealed, the narrator says "Venomoth", but the children shout "incorrect!".