Vampire squid

The vampire squid is a sea creature you catch while swimming in the sea (dive where you see bubbles). Look for a medium shadow. In the northern hemisphere look for it in May – Aug, from 4 PM – 9 AM. It can only appear after you have caught 80 sea creatures. Nook's Cranny pays 10,000 Bells.

A Medium, very fast shadow from late afternoon until morning, after 80 catches. Dive right over it and chase hard; it sells for 10,000 Bells.