This is a lesser known glitch than MissingNO and 'M. However, it exists, and I am reporting it.

It has no attacks (at least at Lv. 7), but is Pokemon Number 250, so this is a great collector's item. To get it:

1. Train a Pokemon so it's Special stat is equal to 197.

2. Walk up to this Gambler (this will not work if you already battled him), 

but Fly away before he sees you. A ! should come up over his head if done correctly.

3. Find another trainer and battle him (Make sure he walks up to you).

4. Find a wild Ditto and force it to Transform into the Pokemon with the Special of 197.

5. Go back to Lavender Town and walk towards the opening to the west. 

The menu should come up. Close it and you'll find a PkMn at lv. 7.

Note: These kinds of Pokemon require you to catch them twice, as if you see its Dex entry, it will turn into a Rhydon.