Seems its a R/S version of our old friend Missingno, here is the info I was able to dig up, the source is pretty legit.
"Ok, this is a weird R/S trick, and apparently, our glitchy friend Missingno has come to haunt us in R/S! This was tested by my friend, and it will screw up your game (theres an 85% chance anyways.) Ok, you have to restart you game. Then, once you do this, beat the game until the island (The one with a cave on the coast, Dew[ford] island). Well, there are 2 fishermen there that you can battle. Battle them in the order that they come. Then talk to the second one after the battles are over. Once you do this, go in the cave. Walk around, and soon you will find a pokemon named ???BADEGG???(its picture is a question mark) Catch him and go to your pc. Withdraw him, and your jaw will drop. Yes, it a bad egg, and its description is (no pokedex entry). An egg that will never hatch. But, that's not all, for the rest of the game, sometimes even after you delete the game, every pokemon that appears will be the ???BADEGG??? pokemon. It seems that R/S has its own missingno and cinnibar island."