From Bulbapedia, the community-driven Pokémon encyclopedia.
The very first Who's That Pokémon?
Who's That Pokémon? (Japanese: Dare Da?) was part of all Pokémon anime episodes from Pokémon, I Choose You! to Watts with Wattson? in the dub and Pokémon, I Choose You! to Hoenn Alone! 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. Additionally, a short hint was added to the screen to aid in the identification of the Pokémon.
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.
Porygon technically makes its only dub appearance while being the Who's That Pokémon? of A Way Off Day Off.
In the Japanese version of Beauty and the Beach, the Dare Da? was Brutella. In the Dare Da? for the Japanese version of The Bug Stops Here, Jessie was dressed as Venomoth. When the "Pokémon" was revealed, the narrator says "Venomoth", but the children shout "incorrect!".
On videos of the anime, the Who's That Pokémon is done like normal, but the answer is given just a few seconds after the question (lacking a commercial break).
In Mewtwo Strikes Back while the Pokémon are being put through the cloning machine, Jessie and James ask: "Who's that Pokémon?" as the Pokémon's images appear. This occurs in both the Japanese and US versions of the film. It is interesting to note that while trying to guess who each Pokémon is, they incorrectly refer to Scyther as Alakazam, among other mistakes.
In one episode, Mewtwo makes an appearance on "Who's That Pokemon", with the voiceover having it say its own name, while in another episode it says "Prepare to battle".
In Double Team Turnover, Ash, when Jupiter's Skuntank appears to steal the Lustrous Orb, asks "Who's that Pokémon?", an obvious reference.