This was all I could find.
With the exception of the eyes and possibly the spikes, it is unknown
what Forretress look like beneath the armor.-Bulbapedia
So,it is still a mystery and gamefreak hasn't revealed anything about it.
The Pokedex can actually be very misleading at times, like on
Weezing's page it explains that triplets have been found, though not
legitimately obtainable; untrue-Fizzcube.