LogFAQs > #884414326

Part of the reason that the Green, Red, and Blue versions were so glitchy is that they did not have any error-checking mechanism to make the game stop if it, say, started reading the player's name data as an encounter table. It was allegedly removed to make room for Mew. Ironically, Mew can only be obtained in Red and Blue in the U.S. by exploiting one of the (many) glitches that can occur due to the lack of an error-checker. Mews caught in the VC-version of the games are flagged as "illegal" and thus non-transferable. Because Nintendo hates fun.

There is evidence that a Shellos-like pokemon existed as an idea as early as in the design stages of Gold and Silver, but it did not actually appear in its final form until 4th generation(Diamond/Pearl).