Did you know that there are Pokemon hidden in Gold/Silver that have alternate colors, aka shiny status? There is no easy way to find them (except for the Red Gyarados in the Lake of Rage) as they appear at random -- but they're out there. If you see specially colored Pokemon, be sure to catch them as they are very rare. You can recognize them by the sparkle effect when they appear in battle, or by checking on the Pokemon Status Screen. Next to the Male/Female symbol, these special Pokemon are marked with three dots.

There are any number of these rare types, including alternate versions of Nidoking (dark blue), Golbat (purple/green), Tentacruel (green eyes), Rapidash (purple), Scyther (orange/green), Hoothoot (yellow/purple), Sudowoodu (green/red), Ho-oh (yellow/orange), and more.