It keeps itself inside its steel shell. The shell is opened when it is catching prey, but it is so quick that the shell's inside cannot be seen.
(From Bulbapedia)Forretress is a large, spherical Pokémon with no visible limbs. The upper and lower halves of its body are capped by bumpy steel plates. The outer plates have zigzagging ridges that open along its back and front, revealing its smooth, red inner shell. The inner shell is punctuated by four cylindrical spikes, each tapering to an open point. Between the spikes, the inner shell opens along a straight line to reveal Forretress's eyes, which are round with large pupils. Whatever else lies inside its shell is unknown.
Forretress can defend itself from enemies by shooting out pieces of its outer shell, as a conditioned reflex rather than a conscious effort. It opens its shell to capture prey, but does so quickly that its interior body cannot be seen. It lives in forests, attaching itself immovably to tree trunks.
(From Bulbapedia)Forretress is based on a bagworm, possibly one inhabiting a discarded shell of a horse chestnut or a walnut. Because of its "shell", and the fact its spikes resemble cannons, it may have been based on a giant clam or other mussels and shellfish with functioning nozzles.
(From Bulbapedia)Forretress and Foretos are a combination of forest and fortress. It may also contain turret, referring to the nozzles on its body that it uses to launch Spikes, and/or ferrous, from the Latin for iron/steel, referring to the steel quality of the shell.