horseradish (n.)

also horse-radish, 1590s, Cochlearia armoricia; the common name preserves the once-common figurative adjectival sense of horse as "strong, large, coarse," as in in obsolete horse mushroom (1866), horse-balm (1808), horse parsley, horse-mussel, Old English horsminte "horse mint." The "London Encyclopaedia" (1829) has horse emmet for a large kind of ant and horse marten "a kind of large bee." Also see radish.

Some nations have used the word bull as an augmentative; the English use the word horse, this being no doubt the largest animal of their acquaintance before the southern breeds of oxen were introduced.

[The Annual Review, London, 1804]