Gastropod

Type	Slime
Environment	The Hallow + Night
AI Type	Hovering AI
Damage	60/120/180 (contact); 50100150 (Pink Laser)
Max Life	220/440/660
Defense	22
KB Resist	20%/28%/36%
Banner	Gastropod Banner
Immune to
Coins	6 SC15 SC
Hurt	https://terraria.wiki.gg/wiki/File:NPC_Hit_1.wav
Killed	https://terraria.wiki.gg/wiki/File:NPC_Killed_1.wav
Pink Laser	https://terraria.wiki.gg/wiki/File:Item_12.wav

The Gastropod is a flying Hardmode enemy and a slime variant that spawns frequently on the surface of The Hallow at night. If the player is within its line of sight, it will open its shell and fire high-velocity Pink Lasers which emit faint pink light.

Like most other slimes, the Gastropod will be passive towards any player with the Royal Gel(Desktop, Console and Mobile versions) accessory equipped.

Notes

Unlike many other flying enemies, Gastropods will enter water to chase the player and will continue firing per usual.
Gastropod lasers will keep traveling as long as they are in sight.
Gastropods have a short firing animation in which they open their shell before firing a laser from their mouth. If they are hit during this period, it is canceled, and their laser attack does not occur.
Gastropods' lasers ignore the Shroomite/Vortex armor's set bonus, able to target players when they are invisible, despite not moving to them while doing so.
Gastropods, unlike most slimes, cannot drop the Slime Staff.
Despite being a slime, killing a Gastropod does not contribute to the Gelatin World Tour achievement.

Tips

Newer Hardmode players may want to avoid the Hallow's surface at night entirely, as Gastropods fire frequently, spawn often, and their lasers are high-speed and difficult to dodge, especially since multiple can appear in the vicinity. In Expert and Master Mode, this is mostly dismissed by the Royal Gel's availability.
Gastropods drop much more Gel than any other non-boss slime, making them useful for gathering Flamethrower or Elf Melter ammo.
Weapons with high rates of fire like the Bee Gun can constantly hit Gastropods, leaving them near-defenseless.

Trivia

Real-life gastropoda are a class of mollusk that includes snails and slugs.
Sometimes Gastropods can be seen flying in the Hallow background at night as ambiance.

The Bestiary entry for the Gastropod: "Touched by the blessing, these nocturnal slimes evolved into a levitating snail-like creature which spits powerful beams of light."

This entry explains that the Gastropods are Hallow-enhanced nocturnal slimes, though it is not clear what connection these have with real-life gastropods. Two possible reasons are that both have very soft and jelly-like bodies, and that both are said to leave behind a trail of ooze-like substances as they move.