Tanoomba

Tanoombas are Tanooki-like Goombas found in both the Mushroom Kingdom and the Beanbean Kingdom. They resemble regular Goombas, except that the Tanoomba has raccoon ears, a raccoon tail, and a leaf on its forehead. Tanoombas have powerful mimicry abilities and are able to shapeshift into different objects and different creatures. However, Tanoombas can always be given away by the raccoon-like features or leaves that appear on their transformations.

Mario & Luigi: Superstar Saga
During the events of Mario & Luigi: Superstar Saga, Tanoombas lived in an area north of Beanbean Castle Town. To attack, a Tanoomba will transform into a copy of Mario or Luigi (albeit a copy with a raccoon tail). After transforming, the Tanoomba will attack the brother who the shape-shifting Goomba didn't transform into (i.e. if a Tanoomba transforms into a Luigi clone, the Tanoomba will attack the real Mario). This attack can be countered with a Hammer strike. Additionally, a Tanoomba can attack by spinning its tail at a Mario brother. The brothers can jump to dodge this attack, but should be prepared to dodge multiple tail strikes. If Game Boy Horror SP is used, it will give out a Green Pepper when defeated.

Mario & Luigi: Partners in Time
Tanoombas live on Thwomp Volcano in Mario & Luigi: Partners in Time. They use a variety of disguises. A Tanoomba can turn into a Toad (with a raccoon tail), a 100 Gold Coin (with the Tanoomba's face on it), a ? Block (with a leaf on top), or even a Thwack (with the Tanoomba's face on it). Once approached, the Tanoomba reveals itself and attacks.

In battle, the Tanoomba uses a variety of shape-shifting attacks (like their Beanbean counterparts, albeit different transformations). First, a Tanoomba can jump high and either turn into a Thwomp (with a raccoon tail) or an orange balloon (with a raccoon tail). If the Tanoomba turns into a Thwomp, it quickly falls and crushes a Mario brother. If the Tanoomba turns into a balloon, it falls slowly, causing damage when it hits a Mario Bro. Both of these attacks can be countered with a Hammer. Additionally, if a Tanoomba twirls when it jumps, it attacks Luigi. If it jumps without twirling, the Tanoomba attacks Mario.

A Tanoomba can also turn into a Spiny shell (again, with a raccoon tail). When it transforms into the shell, the Tanoomba slides into the Mario Bros., circling around and attempting to damage the brothers multiple times. This technique may even poison a Bro. To dodge this attack, Mario and Luigi have to jump as the Spiny shell approaches. Again, if the Tanoomba does a twirl jump before attacking, Luigi is the first brother to be attacked. If it hops normally, Mario is attacked first.

A stronger version of the Tanoomba that has been Shroobified, the Tashrooba, is found later in the game, at Shroob Castle.

Super Mario-Kun
A single Tanoomba appears in volume 36 of the Super Mario-Kun, which is based off Mario & Luigi: Partners in Time. It provokes the four brothers into attacking each other. When it decides to defeat them by attempting to smash them in Thwomp form, after they are worn out, the four brothers realize they have been tricked and team up against the Tanoomba. After the attack, the Tanoomba cries for forgiveness. It first transforms into a Shroob, which the Mario Bros. disapprove. It then turns into a tree, where the Mario Bros. approve, but Luigi has an allergic reaction to it and sneezes the leaves off the tree.

Trivia

 * The name "Tanoomba" is a portmanteau of the words "Goomba" and "tanooki." The tanooki is a real raccoon-like creature that is often involved in Japanese mythology. Like the Tanoomba, the tanooki is a master of disguise and a shape-shifter. Additionally, the tanooki needs to use magical leaves to help it use its powers.
 * The leaf on the Tanoomba's forehead is a reference to the Super Leaf of Super Mario Bros. 3, which transforms Mario into Raccoon Mario. Additionally, the Tanooki Suit is another power-up available in the same game.
 * Tanooki-tailed Goombas that resemble Tanoombas appear in the Nintendo 3DS game, Super Mario 3D Land.
 * Bigger versions of those Goombas appear in Mario and Luigi: Dream Team.