Unused NPCs

Within Terraria there are several NPCs that are unused for one reason or another

Unspawnable NPCs
These NPCs are functional, but do not appear at any time during regular gameplay. They can only only be spawned in by use of a mod or with third-party server commands.

Unused NPCs
These NPCs are incomplete, and cannot be spawned even with the use of mods. Various files or ID names may be present in code.

Unused projectile
These projectiles exist in Terraria&#39;s files, They can only only be spawned in by use of a mod or with third-party server commands. Various files or ID names may be present in code.

It is a leftover after version 1.0.5, when Mud Blocks had a chance to fall, similarly to Sand Blocks and Silt Blocks.