Kraken

The Kraken is a creature that lives in the bottomless ocean of Andros.

Appearance
It looks like a giant squid with a mouth full of sharp pointed teeth.

Overview
According to Tressa, many before have tried to destroy the Kraken but all were unsuccessful and, as seen, it is both immune to magic, due to which the Winx could not do anything to fight against it, and its tentacles grow back immediately after they have been cut.

Season 3
In Season 3, Valtor captured Queen Ligea and imprisoned her in an underwater cave. Valtor asked the Kraken to guard Queen Ligea's jail and the Winx had to fight it to free the her, who was injured in the fight. When Aisha refused to heal herself and healed Ligea instead, she then obtained her Enchantix.

Trivia

 * The Kraken is a legendary sea monster from Scandinavian folklore said to dwell off the coasts of Greenland and Norway and that were said to attack ships.
 * The appearance of the Kraken is unlike the krakens found in legends, which are octopus-like. Instead, it looks like the Kraken in the Pirates of the Carribean movies and has a long squid-like body, fangs around its mouth, orange eyes and is said to be indestructible.