Has anyone documented what a forretress’s interior body looks like? Has anyone ever even seen it?

Yes, although it hasn’t been until recently, with certain advancements in technology, that we could do it. Forretress shells are exactly as hard as you’d imagine, after all.

(Don’t worry, readers. The forretress used for scientific purposes are typically donated to science upon death. Most scientists wouldn’t cut into live forretress.)

In any case, the interior of a forretress is pretty well-documented, but you may wish to consult the texts by Professor Oak on the basics of bug-type physiology if you’re looking for specifics.

Pineco and Forretress

Pineco
The Bagworm Pokémon
Type: Bug
Official Registration #: 204
Entry: This pokémon is famous for its strong resemblance to pine cones, which is not entirely helped by the fact that it dwells primarily in the coniferous forests of Johto. It remains largely motionless as it hangs from trees, moving only to gather bark for its protective coat. Once complete, this coat is sturdy and more than capable of defending its pineco from attacks, but it also weighs more, which tends to result in exhausted pineco falling from their trees. There, pineco can easily be mistaken for fallen pinecones, which causes them to be picked up by humans and used for a variety of purposes, including—unfortunately speaking—fuel for fire. On the other hand, being used for fuel may be a slightly more dignified fate than being coated with glue and glitter and hung from elementary school windows and holiday trees, which happens quite frequently in this pokémon’s native Johto.

Forretress
The Bagworm Pokémon
Type: Bug/Steel
Official Registration #: 205
Entry: The evolved form of pineco, by battle experience. Famed for its hard armor and ability to use the technique Spikes, this nut-like pokémon attaches itself to a tree, where it only emerges in second-long bursts to catch prey. Otherwise, it spends the majority of its time within its impenetrable shell, shooting shell fragments from its outer coat whenever anyone draws near. It is thus valued not only by trainers as a formidable defense but also by recluses—such as this author—as the symbol of ideal living.