Tiny Blue Fish
From Creatures Wikia
This little blue critter lives in the water for most of its life, feeding off of the Fish Nuts that fall in the water. In winter, one or more of the Tiny Blue Fish transform into a Big Fish, if they aren't all eaten by a norn first. New Tiny Blue Fish are born if the Big Fish eats enough food, fruit and seeds and returns to the pond to lay Tiny Blue Fish eggs. Draconian Norns in particular seem to find this fish a tasty snack.
Tiny Blue Fish start out as immobile eggs which are automatically present when Norngarden is injected. Within minutes, they hatch and gain small tails. By the time the season changes from autumn to winter, the Tiny Blue Fish will have grown twice more, reaching their final adult size.