Poisonous frogs are an attempt to introduce into D&D a creature that functions something like a poison dart frog into the game. As such, it is not present as much of a combat threat, but it is a potentially deadly encounter with a poisonous creature.
You look down at your itchy arm, and you see a thumb-sized frog, adorned in bright blues, clinging to your skin.
Hazard: An encounter with a poisonous frog subjects one character in the party to the frog's touch. The character must make a DC 14 Constitution save or be subject to the frog's poison. A failed save means the character suffers from nausea, vomiting, and abdominal pain until their next extended rest, imposing disadvantage on all ability checks and attack rolls.
Countermeasures: An antitoxin is useful in leeching out the poisons, and one can be made from a particular bitter jungle root with a DC 16 Wisdom check.
Uses: The poisonous frog's poison can be harvested with a DC 13 Wisdom check, by roasting the frog over a fire and catching the toxin in a vial. The poison is useful as a contact poison, but is even more deadly as an injury poison. If a target fails the save, they are instantly paralyzed, and go into severe convulsions until death comes in 1d4 rounds. The poison produced by the frog is exotic, and the extract, or the frog itself, may go for a fairly high price in certain assassin markets.
World: Poison frogs live in the jungles, and are uncommon there.
Story: Poison frogs can be used by jungle natives to coat their hunting (or war) weapons, and assassins may value or use the poison.
Mechanics: Poison frogs are quite deadly under the right circumstances (when used as a weapon), but are otherwise simply a complication that can hurt a particular party member.