Giant Clam

This was the personal summon of the Second Mizukage.

Personality
Unlike other summons, this clam seems to lack its own personality and is completely loyal to its summoner regardless of the intention, even if it means fighting Kirigakure shinobi.

Appearance
Aside from its massive size, it resembled an ordinary clam. Several prominent dark lines encircle both the upper and lower portions of its shell, while the Kirigakure symbol is inscribed upon its top.

Abilities
The creature is capable of producing a mist with which its summoner can create a deceptive mirage that completely rendered observers unable to accurately locate the position of itself or its summoner, after firstly opening its shell and exuding a mist from its siphons throughout the duration of the technique. Its shell was very sturdy, being able to resist Ōnoki's initial attack. This creature also seems to have an extraorindary long life span as it was used by Gengetsu when he was alive and survived until the Fourth Shinobi World War.

Shinobi World War Arc
During the Fourth Shinobi World War, after the Second Mizukage was reincarnated by Kabuto Yakushi to fight for Akatsuki, it was summoned to the battlefield in a confrontation against the shinobi of the Fourth Division. As the clam opened its shell, it envelops the area in a mirage that rendered all the attacks the shinobi dealt ineffective, as they remain unable to locate the actual position of either the creature or the Mizukage. Despite the Mizukage's advice to attack the clam standing directly behind him, their attempts continue to fail, resulting in utter confusion amongst the shinobi. After Gaara and Ōnoki arrived, the real clam was located through Gaara's Concealed Sand Picture Cat technique. Ōnoki initially tried to attack the clam with his Dust Release but is unsuccessful because of his dwindling chakra reserves. The clam is then attacked by Ōnoki's Earth Release: Fist Rock Technique which did little to no damage to it until he uses the Earth Release: Ultra-Added-Weight Rock Technique to destroy it. Due to this, the mirage disappears.

Trivia

 * Although simply called 'giant clam', the kanji is actually the name for a mythical giant clam and is usually pronounced as 'shin' in Japanese ('shèn' or 'chèn' in Chinese). This shin is actually a kind of aquatic, shape-shifting dragon in the form a giant clam. It is said to be able to cause mirages and Fata Morganas. In fact, its name has become almost synonymous with 'mirage': in Japanese, the word for 'mirage' is.
 * This and other references to mirages relating to the giant clam are a further allusion to the Second Mizukage's use of genjutsu.
 * The giant clam adds to the list of summons who are seemingly allied to a single ninja village given the Kirigakure symbol on its shell.