Sea pig

The sea pig 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 Nov – Feb, from 4 PM – 9 AM. It can only appear after you have caught 80 sea creatures. Nook's Cranny pays 10,000 Bells.

A small but very fast shadow from late afternoon until morning, after 80 catches. It is easy to overlook, yet it sells for 10,000 Bells, so chase every quick little shadow.