Sounds like a Sam Kieth comic:

A 9-year-old Minnesota girl found a five-legged frog with 23 toes near Stewartville, Minn., according to a report. … Experts say the animals are particularly sensitive to pollutants, which can be absorbed through their skin. Because of that, deformed frogs are considered a sign of environmental problems.

local6: Minn. Frog Found With 5 Legs, 23 ToesSome Experts Disturbed By Discovery

(via Last Word Blog).