Slug

are the trademark summon of Tsunade, one of Konoha's legendary Sannin, as well as her student, Sakura Haruno. According to Orochimaru, they are summoned from the Shikkotsu Forest.

Overview
Slugs' offensive abilities involves shooting acid from their mouths, similar to the way toads use oil. For defending others, the slugs have the ability to encapsulate themselves around an individual's entire body. This essentially makes them a big cushion for the person inside, protecting the person from physical collisions. Slugs have also been shown to be able to divide themselves into many smaller replicas, though this ability may be unique to Katsuyu due to its size. Katsuyu can also absorb chakra from its summoner and even control it to some extent, as shown when Tsunade transferred her own chakra to Katsuyu in order to heal the citizens of Konoha when Pain attacked.

Known Slugs
The personal summon of Tsunade, as well as her student Sakura. With its great size and abilities, it fought side by side with the other two great summons of the Sannin, Manda and Gamabunta, and later helped to save and protect the villagers during the invasion of Pain.
 * Katsuyu

The Six-Tails.
 * Saiken

Summoners
Also known as, Tsunade signed a contract with these slugs at a very young age. As she got older, she became more renown until she became synonymous with her summons, hence the moniker. She made use of them during the Second Shinobi World War, most noticeably against Hanzō the Salamander and then later after the start of the series against Orochimaru during the battle of the Sannin. Her most notable use of slugs was during the Invasion of Pain, where she summoned Katsuyu and had the slug attach itself to every member of the village so that she could heal and monitor them. It was because of this that the members of the village survived the attack on the village and the number of casualties was kept at a minimum. More recently, she has summoned Katsuyu once again to act as a mediator, relaying information to and from the headquarters of the Allied Shinobi Forces after she and A went to the battlefield.
 * Contract::Tsunade

In the Birth of the Ten-Tails' Jinchūriki arc, Sakura is able to summon Katsuyu to the battlefield, where the slug was asked to split, and attach herself to the members of the Allied Shinobi Forces so that she could heal them.
 * Contract::Sakura Haruno

Known Slug Abilities

 * Chakra Transfer Technique
 * Slug Great Division
 * Tongue Tooth Sticky Acid

Trivia

 * The acid-shooting powers are an exaggeration of real-life slugs' ability to regurgitate digestive acids in self-defence.
 * Katsuyu has been the only slug seen summoned by Tsunade, as well as Sakura, in the series to date. The other slug seen in the series is Saiken. Kishimoto has, however, drawn three other slugs: one on the the cover of chapter 4, another on the cover of chapter 153 and the third in the colour spread of chapter 162.