A Giant Shelly is an Enemy found in the Cavern layer. They attack by curling into their shells and hurling themselves at players, though they will stall in midair and drop to the ground if they are damaged during their flight. Giant Shellies attack very quickly and deal high damage for the stage in the game where they are found.
Giant Shellies are unique in that their ability to spawn is connected to two other enemies: the Crawdad and the Salamander. Of these three enemies, a world will choose two types that it can spawn, leaving the third inaccessible. Giant Shellies will never spawn in a world that features both Crawdads and Salamanders together.
Notes
Giant Shellies are essentially smaller Giant Tortoises and Ice Tortoises, found at a point where a player has fewer options for movement and evasion and must engage them at close range or with slow, weak projectiles.
Trivia
Like their aforementioned analogs, Crawdads and Salamanders, Giant Shellies have multiple colored variants. Also like said analogs, these differently colored variants are statistically identical, unlike various Zombie varieties.
The Giant Shelly's name may be a reference to Scott Lloyd Shelly, who composed much of Terraria's soundtrack.
The Giant Shelly could be a reference to a variant of sea snail that has a spiny shell.