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 this happens so quickly that its interior body cannot be seen. It lives in tropical and temperate forests, attaching itself immovably to tree trunks.
In Entei at Your Own Risk, Brock's Pineco evolved into a Forretress in order to save a group of Pokémon at a hot spring from Team Rocket. When Brock went to Hoenn, the only Pokémon he brought with him was his Forretress. Brock later left Forretress at home when he headed to Sinnoh.
In A Bite to Remember, Brock's demonstration featured a Forretress evolving from a Pineco.
A Forretress was seen in Crossing Crobat, under the ownership of Janine. She used it to escape from the security guards with its Spikes after she tried to steal a Big Pearl from the Goldenrod Museum as a security test.
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.