It is not possible to catch a wild Turtwig on Pokemon Diamond. It is generally impossible to find and catch Starter Pokemon in the wild. This was a mechanic designed to encourage player interaction for trading.
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you can't catch a wild turtwig. I know it sucks. You must breed one or trade with another turtwig owner.
Turtwig is a starter Pokémon, it can't be found in the wild. If you choose another starter, then the only way to obtain it is to ask a friend who started with Turtwig to breed it and trade the egg with you.
Turtwig is a Hoenn region starter Pokemon. Thus, it cannot be found in the wild for capture in HeartGold. However, after defeating Red at Mt. Silver, you have a chance to get a Turtwig. Go to Silph Co., talk to Steven (the Hoenn champion), and choose the green stone to get a Turtwig.
Your Mom! Catch shiny Pokemon like a non noob by chaining!
You can't catch Cranidos in the wild or from a special event. You'll need to trade for it from Pokemon Diamond, Pearl, or Platinum.