echidna.

Coined in scientific literature around 1811. Probably from Ancient Greek ἔχιδνα (ékhidna, “snake, viper”) via Latin echidna. Compare ἐχῖνος (ekhînos, “hedgehog, etc.”). However, this sense is problematic (unless it is a reference to the ant-eating tongue). The name perhaps belongs to Latin echinus (“sea urchin, hedgehog”) from the aforementioned Ancient Greek term's alternate sense of "sea-urchin" (also "sharp points"), which Watkins explains as "snake-eater", from ἔχις (ékhis, “snake”), though it may actually be from Proto-Indo-European *h₁éǵʰis (“hedgehog, hedgehog-like animals”). The 1810 Encyclopaedia Britannica deduces thus the animal's alternative name as "porcupine ant-eater". Alternatively, and perhaps more likely, the name refers to Echidna as the name of a serpent-nymph in Greek mythology, "a beautiful woman in the upper part of her body; but instead of legs and feet, she had from the waist downward, the form of a serpent", in which case the animal was named for its mixed features (early naturalists doubted whether it was a mammal or amphibian). Ultimately, the etymology may be from a synthesis of all the roots above. (From OED.)