Goozims are large, brown, hairy, seemingly carnivorous creatures native to Drusselstein.
Goozims appear to be dangerous but capturable as a game in Drusselstein called "Poke the Goozim with a Stick" shows one being locked up and poked by civilians. Mr. Doofenshmirtz was the winner of "Poke the Goozim with a Stick" and received a Spitzenhound as a prize, which he named "Only Son" ("Got Game?").
This appears to be a traditional and popular game in Gimmelshtump. The game is straightforward, in which people merely poke the Goozim with a stick. It is unknown how one wins this game, but prizes may be awarded based on the Goozim's reaction to the poking. Mr. Doofenshmirtz appeared to use a particular stick with a badly-made sackcloth horse-head on it; this may have been an official Goozim-poker or just Mr. Doofenshmirtz's favorite stick.
Goozims may be sentient or semi-sentient because if one listens closely when Mr. Doofensmirtz pokes it with a stick, it sounds as if it is yelling "OWWWWW" ("Got Game?").
Goozims don't appear to be very strong for their size, as the Goozim cage depicted in Got Game? doesn't look very strong. However, in the 2nd Dimension, the Goozim cage is shown to be made of iron, indicating that in the 2nd Dimension, Goozims may be much stronger than their 1st Dimension counterparts or the iron could just be the only mineral there to use (Phineas and Ferb The Movie: Across the 2nd Dimension).
According to Phineas Flynn, an individual Goozim is approximately the size of a two-car garage. However, as this observation took place in the 2nd Dimension, it may only apply to 2nd Dimension Goozims.
Goozims appear to be composed almost entirely of hair, similar to the Looney Tunes monster Gossamer. This was shown when the 2nd Dimension Goozim leaped through a portal in pursuit of its 1st Dimension prey. There was only room for a small cylinder of Goozim to go through the portal, but when it passed through the portal, it was shown to be still alive.