As I mentioned further up the thread, there are numerous "strains" of
Tholians in the Star Fleet Universe. But the dominant "Pillar" strain has no legs, and possesses a limited means of hovering above the ground instead.
Actually, if you want to see a preview of how the Tholians would be detailed in
Prime Directive (along with a bunch of other fiction and background articles of note), there is a set of files sieved out of various issues of
Captain's Log and grouped in one place in
Away Team Log.
ATL is available right now on
Warehouse 23 and
DriveThruRPG, while the print edition should go to press in the near-to-mid future. (You can see a list of documents in this file
here.)
Away Team Log is ruleset-neutral, so should work just as well as a resource for
Mongoose Traveller players as it would for those of
GURPS4e and
D20M. (There was talk at one point of a series of supplements supporting the
ATL material in each ruleset, but I don't know what the current story is on that front.)