Does your world have enough amphibians?
Some of the hoppers I can ID in the local vicinity include the green treefrog (Hyla cinerea), the southern toad (Bufo terrestris), and the squirrel treefrog (Hyla squirella). Based on the number of distinct frog calls at night, there’s probably a few more I haven’t seen.
(Yes, I realize they’ve apparently changed up scientific names recently. The ones up there are the ones I learned and that are in my ID guide.)
Frogs and toads do many things. Splash. Hide out in cracks, crevices, and flowerpots to startle you at the most awkward moment. Make enough noise calling at night to make you stuff the covers in your ears. Stop you dead in your tracks, blinking, as the ground seems to move – only it’s a scattering of toads tiny enough to perch on your pinkie nail, all hopping away.
But first and foremost, frogs eat bugs.
I use that term specifically. First, frogs could care less if it’s a true insect, spider, millipede, or what have you; if it doesn’t move fast enough, it’s dinner. Second – that quality of eating bugs is what makes them a major ecosystem component, transferring energy up the trophic levels to more warm-blooded vertebrates, including ultimately us. To give just one example, marshy areas grow grasses and plants, which feed bugs, which feed frogs, which feed everything from herons, raccoons, foxes, and gamefish, not to mention stray cats and Frenchmen. It’s not energetically advantageous for a warm-blooded critter the size of a raccoon to be snapping at flies, and they don’t have the stomach (literally) to eat marshgrass. Frogs? They can hunt hoppers all day, and get enough food to get by.
Real worlds have trophic levels. Areas with enough moisture and nutrients tend to produce a lot of biomass; areas missing one or the other, much less. And areas lacking in both – say, Antarctica, or the Australian Outback – tend to be very, very hard places to make a living. Note that Emperor penguins only pull it off by getting their food from a different ecosystem; the ocean.
If your world is pulpy enough, and the monster ticks enough “sleeping dragon!” boxes in the psyche, maybe nobody will care if you put your turtle-snake-magical beastie in a deserted cave with no food coming in. But if your heroes are suddenly battling griffins in the middle of a burning desert, they’re going to expect some kind of oasis within flying distance. And where there’s water, there’s generally frogs. Sometimes from eggs carried on waterfowl feet; sometimes from adults that managed to escape being some bird’s carryout meal.
If there’s water that doesn’t support frogs, your characters may want to think twice about drinking it. And run tests for toxins, if they can.
Every world needs to be built from the ground up. Make sure when your characters yell “Frog!” something jumps.