Umbrella octopus

The umbrella octopus is a sea creature you catch while swimming in the sea (dive where you see bubbles). Look for a small shadow. In the northern hemisphere look for it in Mar – May, Sep – Nov, at any time of day. It can only appear after you have caught 40 sea creatures. Nook's Cranny pays 6,000 Bells.

A small, fast shadow found at any hour in spring and autumn, after 40 catches. Small shadows are easy to dismiss, but this one sells for 6,000 Bells, so check them all.