I've just written the question Are there “Space Jellyfish” in Florida? Would they be possible?
The phenomenon is described in Wikipedia as follows:
A space jellyfish (or jellyfish UFO; also rocket jellyfish) is a rocket launch related phenomenon that frequently is misidentified as an alien UFO. The phenomenon is caused by sunlight reflecting off the high altitude rocket plume gases emitted by the launching rocket, at pre-dawn and post-dusk, the twilight plumes. The observer is shrouded in darkness, while at high altitudes, sunlight is able to reflect off the exhaust that is being lit before dawn reaches lower altitudes or after dusk has already left lower altitudes, due the curvature of the Earth and its rotation causing the day-night cycle. This luminous apparition is reminiscent of a jellyfish.
In the title I used the plural form space jellyfish because it sounds humorous, as if there could be a school of such fish. But in the body of the question, I use space jellyfishes because I feel that this is more likely correct.
But I don't know why I feel this way nor how I made this choice.
Is there a grammatical rule that I'm using intuitively without realizing it? Is there some a priori reason that would tell us that the fishes form would be the correct choice, presuming that it is?