Lapras!
On Fridays (in-game time) in Union Cave, a Lapras will appear in a room witth a lake. The lake where Lapras is found is hidden deep within Union Cave, but it isn't much of a challenge to reach. If you capture or defeat it, it will return again next Friday, but no more than once per day.
If you need more help, we've got more Pokemon Gold cheats and also check out all of the answers for this game
Comments for this hint