Whale shark

The Whale Shark is a very rare sea fish in the introduced in. The Whale Shark has a "huge (finned)" shadow size when seen in the water.

In
After players donate a Whale Shark to Blathers, it will appear in the section for fish and sea creatures, specifically in the large tank in the north-eastern room. The exhibit has this to say about the whale shark:

In
During a Fishing Tourney, when given a Whale Shark, Chip will say the following:

Real-world information
The whale shark is the largest extant fish species, feeding primarily on plankton and other small organisms.