Aether Tetra
The Aether Tetra (an Aether Creature) is a Tank Fish that is part of the Seasonal Content, and it can only be caught (using the Fishing mechanic) when the comet is visible in the sky of New Britannia (which is several months per year in real-life time, often May through September).
You can place an Aether Tetra in a Fish Bowl or a Fish Tank as decoration.