The Tropic Planet is a scrapped location in Serious Sam: The First Encounter.
The Tropic Planet would presumably be a planet filled with tropic plants. Based on this, it might've looked like the South American portion of Serious Sam: The Second Encounter, as it is in a tropic area. It consists of two levels; End_Arena and Tropic. The unusual level flow (what appears to be the final area before the actual planet) is likely to assist with beta testing.
There appears to be no elemental enemy that is directly tied to the planet, based on their names. The Air Elemental might've been the elemental enemy for this level, as its name doesn't match any of the planet's names.
The Tropic Planet was removed when all of the post-Egypt story arc content was removed from The First Encounter. There are no traces of it in the final game.