The only secret units I know of are the Yubtseb elephants. Which are basically the Mumakil/Oliphaunts from L.O.t.R.

CA did put these units in the game however as a rebel tribe called the amazons can occasionally get them

I did a search of export_descr_unit, which is the list of game units, for the terms panzer, rocket and tank and it brought up no results.