2 hit die, armor class 7 'frog mutations' created "unknowingly by the Master Alchemist Vollan."

more. . .

"During his experiments to produce a new acid, certain untreated chemical wastes from his laboratory found their way into the drainage system with eventual contamination of nearby streams and ponds. Fortunately, no other forms of life were affected besides the frogs."

The mutations?

The tough leathery 'outer skin', razor-sharp teeth, and venomous fangs.

Two mechanical things of note: The creature is considered to have one hit die if fighting outside of water. Also, the creature is immune to normal edged weapons and clubs "etc.", having only vulnerable spots being eyes and softer underbelly. 

I'm guessing the game of "where do you attack the monster?" was much more popular back in the day. It's also pretty easy to see why this didn't make the cut. Giant frogs have an armor class of 7, and killer frogs have fangs. (From White Dwarf #2)