It was on pokémon stadium 2 and gen 2. 0 catch rate due to item used/removed, nicknamed due to a "bug"(?) in stadium 2 (and I think stadium 1, but I have yet to test it) not reading the full name, but stopping at the first 0x50 terminator. When pokemon comes back to the GB cartridge data present alredy in the box/party gets "added" to the remaining characters. This doesn't happen with OT.
EDIT: also, YaniM told me that the mew had been in stadium 2 and gold before I even asked about it. When trasnfered to gen 2 mew holds a bitter berry. The mew was used and brought up to level 100, being