A terrain is symbolized as a circle with one input slot (white circle).
The terrain icon can not be deleted. You can only change it's properties.

You can create the following types of terrrain from pG.
Landscape PNG's.
Uses for heightfields, bump maps, colour maps etc.
Landscape Meshes.
A displaced plane, in Wavefront OBJ or POV-RAY mesh2 format.
Planet PNG's.
Texture maps, normals maps etc. Suitable for UV mapping.
Planet Meshes.
A sphere where the points have been dispaced along the vertex normals.
Terragen Terrains.
Terragen ter format files.