HeartGold- Its entire body is shielded by a steel-hard shell. What lurks inside this shell is a total mystery.
I'm wondering what's in it's shell, has it been revealed
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.
