This is a test that I have made to learn the basics of instancing in houdini.
The spider animation was defined between POP's and CHOP's; Then thanks to the instancepoint() function, all the animation attributes per agent point can affect the spider rig properly.
For the terrain effect, an SDF representation was used to keep the agents over the terrain surface (a much better alternative to the ray sop).
Finally the displacement of the agents over the terrain is fully editable with the painting of attributes over the terrain.
The hip file example is here: sidefx.com/index.php?option=com_forum&Itemid=172&page=viewtopic&t=25840