This creature is most likely based off the popular "rod" optical phenomenon (also known as "skyfish", "air rod" or "solar entity"). When recording on video, sometimes odd moving streaks appear resembling the Sky Fish in the game. Popular folklore depicts them as the result of bugs flying past: the central rod is the bug's body and the wave-like perimeter is the bug's wings beating faster than what the video recorder can capture. When it was first noticed in the early 1990s, few suggested this theory, and more unusual ones, such as rods being things from another dimension, were brought up.
The Sky Fish is a strange creature that moves at very high speed. At first glance, the Sky Fish may resemble an ectoplasmic formation, with a light-blue ghostly body and emitting a purple-colored aura around it. However, on closer inspection, the Sky Fish has a head with yellow eyes and a tail-like formation on its rear. Its body resembles that of a Mollusca, with a mantle-like moist body.
It can be seen flying by at an absurd speed in only two rooms in Castlevania: Aria of Sorrow, both located in the Underground Reservoir (one of which is the room with the Big Golem). With good timing, Soma can use the Chronomage's soul ability, Time Stop, to slow it down and make it able to be damaged. Timing really is the key to killing it, since there will be a few seconds delay between when Soma enters the room and when it passes by. It won't appear if Time Stop is used too early, and if used after it appears, it will be too far away to attack when it slows down (even slowed down it is faster than most enemies in the game).
The Sky Fish's soul has the highest drop rate among non-boss enemies. Without any LCK Boost equipment, Soma will have at least 15% chance to get the soul. This value can be increased to 35% if the Soul Eater Ring is equipped.