Aquis, called Seadra (シードラ) in the Japanese manual, is an enemy from the 16-bit version of Sonic the Hedgehog 2 found in Oil Ocean Zone. It is a flying seahorse that moves swiftly and quickly fires quick black bullets at the player. They are a pain to get around, so to try to hit them with precision.
In both the Nick Arcade and Simon Wai prototypes, code and sprites exist for another type of Aquis which behaves erratically and shoots oil balls at Sonic when approached.
They return in the Wii U/PC version of Sonic Lost World where they can be found in Frozen Factory Zone 1. However, instead of shooting oil at the player, they will instead use a vacuum attack that will steal the player's Rings. Defeating them will release the Rings they stole.
They return in Sonic Mania, again in Oil Ocean Zone with their original attack pattern from Sonic 2.