finding one
There is nothing to trigger. Ordinary jungle frogs occasionally come out as this one instead, at a low but not painful rate, and only one can exist in the world at a time. It has almost no health and no defense, so anything in the jungle can kill it, including a stray minion and including you by accident. A Lifeform Analyzer earns its slot here because it calls the frog out by name. Without one you are inspecting every frog on the riverbank looking for a hat.
the net does not work
This is the one critter a Bug Net will not catch. Swing at it and it teleports somewhere else nearby, or vanishes in a puff of smoke if there is nowhere valid to land. There is no cage and no way to keep one in a jar, so stop thinking about it as a catch.
what to bring instead
Purification Powder. Throw it over the frog and it becomes the Mystic Slime, a town pet that then lives in your houses like any other. The trade is one way and permanent for that world: afterward the frogs stop appearing entirely. That costs you nothing, since the pet comes back on its own if it is ever killed and you never need a second frog. Carry the powder before you go looking, because the sighting and the conversion have to happen on the same trip.
trivia
- It holds the highest internal NPC id in the game.