Entities are described in the ent library - ops/data/lib
lib has a few subdirectories for common things - tex,meshes,particles,heightmaps...
the other directories each describe a game entity type
Files
ent.tcl
describes the entity tree
adds aspects
sets variables
vis.tcl
describes the scenegraph that is created for each entity
links to /node/entname
particle effects go here
node.tcl
describes the static scenegraph
most visual stuff goes here
node / mesh / etc.
data
Directory that usually holds the meshes / textures for each entity
Creating an Entity Type
Best way is to copy an existing directory - then change things to suit you. Be sure to rename everything in ent/vis/node and check the data files - they are often named after the ent