The word mansuetude refers to a quality of gentleness, mildness, or tameness. For a new rider, mansuetude is a very desirable quality in a horse.
Mansuetude is a noun form of an adjective that is obsolete: The word mansuete meant "gentle, meek, or tame," but it's no longer used. The word's Latin roots are manus, meaning "hand," and suescere, "to accustom." In literature, you might see the word used to describe gentleness in a person or imaginary being: "The giant looked frightening, but he displayed a surprising mansuetude." The word can describe things in nature, too: for example, "the mansuetude of the morning breeze after the previous night's storm."