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 you. They are a pain to get around, so to try to hit them with precision.
In both the Sonic the Hedgehog 2 Nick Arcade Prototype and the Sonic 2 Beta, code and sprites exist for another type of Aquis which behave erratically and shoot oil balls at Sonic when approached.
They return in Sonic Mania, again in Oil Ocean Zone.