ゥ is a dual-type Normal (hex 8D)/Ground (hex 84) glitch Pokémon in Pokémon Red and Blue. As this was one of the first glitch Pokémon documented in the English Pokémon glitch community that contained Japanese text in its name, it was sometimes called "Japanese Symbol" by those unfamiliar with katakana writing system.
It will always freeze the game upon encountering in the wild, so it cannot be obtained through the Mew glitch unless encountered in Pokémon Tower as a Ghost. Its cry is a slowed-down version of Psyduck's.
The back sprite is volatile the first few times it is sent out and will likely crash the game until it becomes constant. However, the result is random.
This video is not available on Bulbapedia; instead, you can watch the video on YouTube here.
The video above showcases the back sprite's behavior.
Trivia
Setting addresses $A70F and $B30F to 0x08 will prevent the game from crashing, but it will load a glitch trainer instead of ゥ.
This glitch trainer will send out a level 0 'M (00) with a flipped sprite.
Since ゥ's sprite is so large, it manages to heavily corrupt the game; party Pokémon and items are affected by this. Attempting to attack will most likely crash the game.
This glitch Pokémon article is part of Project GlitchDex, a Bulbapedia project that aims to write comprehensive articles on glitches in the Pokémon games.