Data IDs

A Mob/Item is missing from the list! What can I do?
This means that either the item/monster is saved in the Terraria client with its name, instead of ID, or it was added in a recent update and the page hasn't been updated yet. For example, items such as Gold Pickaxe and monsters such as Pinky and a few other colored slimes, are saved with their name, and can't be indexed with an ID.

What do I need these IDs for?
Basically, these IDs are used to index monsters or items in Terraria.