Seaweed

You get a turtle, and a turtle cannot keep up with a player. Let it fall behind and a propeller unfolds from its shell so it can fly after you, which is the entire reason anyone wants this one. It is one of the better looking pets in the game for a fairly small time investment.

It is a jungle chest item, so the underground jungle shrines and living mahogany trees are the places to dig. Fishing the jungle gives you a second route through crates if you would rather cast a line than tunnel.

The flying turtle is a nod to Tank from My Little Pony: Friendship is Magic, right down to the propeller solving the same problem. The community suspected it for years before Redigit said so outright.

Worth flagging: fishing junk shares this item's exact name. Two unrelated things in your inventory read identically, so check the tooltip before you throw one away.