Infested Kraken is result of what happened when Nastasia/Nautilia the Trident Of Marrow once saw a horror movie about an octopus and she got an octopus for dinner. In the end, she made a movie true, as the infested kraken is one of her notable units to send against her foes.

As seen, Nastasia as a Zerg woman is affiliated with aquatic life, especially crustaceans and cephalopods. Therefore, nothing surprising should about that she has a crab-like units capable of evolving even further.

(Zerg belongs to Blizzard)