Looking at these 6 artistically, you can hardly blame anyone for confusing them. Before I started this topic I would have confused a few. And this isn't even including the grung or grippli!
The hezrou is the oldest of the bunch, however unlike all the others from this selection, it lacks any defining froggy features justifying a froggish appearance.
1. Hydroloth/Hydrodaemon: This yugoloth has the most striking artistic similarity to #3 (Hezrou), yet it has key froggy features that #3 lacks: it's Amphibious (inhabiting the River Styx) and has Watery Advantage. It dates back to 1e's
Monster Manual II.
2. Banderhobb: Introduced in 4e's
Monster Manual III, this monster from the Shadowfell is a faerie tale boogeyman which leans on these froggy feature: it's stealthy with Shadow Stealth, it has a big mouth to swallow you with, and a tongue to reel you in.
3. Hezrou: This demon traces back to the OD&D's
Eldritch Wizardry as a Type II demon, but unlike every other monster in this selection it lacks any defining froggy traits in either its stat block or the rest of its monster entry.
4. Froghemoth: Introduced in
Expedition to the Barrier Peaks, this alien monster has multiple froggy traits, including: it's Amphibious, it has rubbery skin manifested as lightning resistance and Shock Susceptibility, it has a big mouth to swallow you with, and a tongue to reel you in. With a distinctive triple-eye stalk and tentacles, it earns its name.
5. (Green) Slaad: First appearing in the 1e
Fiend Folio, these extraplanar monsters from Limbo have one froggy trait: they undergo a birthing process which involves evolution from a (horrific) tadpole to an adult. They're more like xenomorphs than frogs, and their stats otherwise don't evoke frogishness (e.g. they could just as easily be drawn as green, red, or blue wolverines and their stats would the same), but at least they have the tadpole biological process as explanation for their froggish appearance.
6. Bullywug: The only monster native to the Material Plane among this selection, bullywugs were introduced in the 1e
Fiend Folio. They possess the following froggy traits: they are Amphibious, they Speak with Frogs and Toads, they are stealthy with Swamp Camouflage, and they can perform a Standing Leap. Known for riding giant frogs/toads, they earn their place among the most "froggy" of the frog monsters.