In “Pokémon GO,” the Pokémon with a violet body and a gold formation on its front is Starmie. Its design resembles two overlapping starfish, with the forward section surrounding a distinctive central core.
Starmie is the evolved form of Staryu and is listed as Pokémon number 121 in the National Pokédex. Unlike Staryu, Starmie has a second, semi-attached star-shaped body on its back. That rear section can rotate independently, helping Starmie move through water like a propeller.
The gold structure on its front encloses a jewel-like core, usually shown as red. Pokémon lore says the core can glow in all seven colors of the rainbow. Several Pokédex entries connect those changing lights with communication and with electrical waves sent toward outer space.
Players sometimes confuse Starmie with Staryu because both are starfish-like Pokémon and both have a central jewel. Staryu has a single star body, while Starmie’s extra rear star and gold-framed core make the description in the question specific to the evolved Pokémon.