Shadowfish

The Shadowfish are mysterious ghost-like creatures that according to Lady Olivia they are otherworldly creatures that were brought to Amphibia through the Calamity Box and kept in Andrias' basement.

Appearance
The Shadowfishes are translucent fish-like creatures, similar to germs in a way, all have tentacles or antannaes on their bodies as their eyeballs are clearly visible in their bodies.

When reflected by mirrors they show their true appearance, loosing their translucent appearance and just being blue fragile creatures.

Background
These creatures were kept inside the basement of Newtopia Castle, along with a Moss Man, in order to be studied for their nigh-impervious durability. This research led to the creation of the entity that lords over King Andrias.

In the present, while exploring the basement, Anne, Marcy, Sprig, and Polly encountered the Shadowfish. The kids discovered they can be defeated by making them look at mirrors to make them from phantom form to solid for hitting them, and they forced the Shadowfish to retreat after enduring many hits.

Trivia

 * Some Shadowfishes seems to appear in Andrias' hidden mural in an unknow city like location in flames, hinting that probably their world has been already conquered or destroyed by Andrias or his ancestors.