The Grateful Frog is a harvestable that can be obtained in the Frontier. It can be found from various sources and locations throughout the Frontier including but not limited to:
The Grateful Frog is a green frog humanoid-like figure trapped in an iron cage. Above the cage is a hook where the cage is hooked and/or attached on an object, terrain, and/or props located in the Frontier. Players can also exchange 5 Grateful Frogs for one of many potential items available during the Frog Quest, or exchange 1 Grateful Frog for 500,000 gold during the Slayer's Tasks. If the player has no purpose for the Grateful Frog, it can be sold for 100,000 gold.
List of Locations
These are the known locations of the Grateful Frog. The locations of the Grateful Frogs varies and not all spawns are known to the public. A Grateful Frog has a 45% chance to spawn at any of these locations every 15 minutes.
If you prefer video footages to understand and know where these Grateful Frogs tend to be found at, click or press this video to start watching discord user ExtremeNinja777's (fantastic) video about how to farm Grateful Frogs. This video is explained well to allow beginner players to understand and for the dedicated, experienced players to keep in mind.
Map
If you prefer to view a map to see all of the known spawns of a Grateful Frog, click on the map below to see it in full view. Thanks to 1rubberducky2 for doing this out of courtesy~!
The map showing all, if not, most of the known spawns of a Grateful Frog.
Trivia
Rather than seeing [E] Harvest, the player will see [E] Rescue, as implied by its description where the Grateful Frog is "saved" by the players until eventually they are sold to a selling vendor or turned in to the Blue Frog.
According to the developers, the Blue Frog buys the freedom of the Grateful Frogs.