First Appearance: IPTSF Text 47
A Tannenbaum is a treelike non-humanoid, forest-capable Unit, similar to Gumps, but nastier. They have the appearance of animated Pine Trees and can be decorated with various objects, like garlands, bells and battle ornaments, which aid it in combat.
Tannenbaums have very low move. IPTSF Text 48
Tannenbaums appear to be animated units, but it's possible they have a special ability allowing them to hibernate as trees.
Real World References
Tannenbaum is German for Christmas Tree, which corresponds to their appearance.