The tadpole hunter, Tyrannopapilio ranunculiphagus, is a riodinid butterfly native to southeastern Venezuela and the Guianas. It is the only known predatory butterfly. Its diet consists almost exclusively of tadpoles of Spix's Snouted Treefrog, Scinax nebulosus.
Adult butterflies perch on vegetation around temporary waterbodies and capture tadpoles with their harpoon-like proboscis. The prey is instantly paralyzed with a strong poison secreted by the salivary glands.
Really? No, not really. April Fools! Butterflies are pretty cool, and they display a number of interesting behaviors, but hunting tadpoles is not one of them. Although the butterfly proboscis may appear somewhat harpoon-like, it's way too flimsy to put in the service of subduing prey. It works for sucking up juices, but is generally not of much use as an attack weapon (but see below).
That all being said, the idea of a carnivorous butterfly is not as far-fetched as it may seem. Some butterflies in the lycaenid subfamily Miletinae are predatory in the caterpillar stage (see Pierce 1995), e.g., the Harvester, Feniseca tarquinius, and the Apefly, Spalgis epius. Also, adult butterflies can sometimes be seen feeding on carrion, because they are generally attracted to rotting organic matter as a source of protein. Finally, there are of course the vampire moths in the genus Calyptra. Distantly related to butterflies, these noctuid moths have a modified proboscis that has strongly sclerotized erectile barbed hooks at the end and is used for piercing both thick and hard-skinned fruits such as peaches, plums, and citrus fruits. And wouldn't you know it, some Calyptra moths actually can use their proboscis to pierce mammalian flesh and feed on blood.
So you see, a story about a tadpole hunting butterfly is not quite as crazy as one might think.