Pineco and Forretress are both based on bagworms, a type of caterpillar that covers itself with natural materials for protection. Pineco is closer to the real thing, since bagworms look like pinecones when they’re hanging from branches. I think it’s implied that Pineco’s constructed covering turns into a hard shell when it evolves. The shell is like a protective fortress, which explains Forretress’s name.
So, Forretress is a bagworm, if a bagworm’s defensive covering was a fortress.