Definition of "mettlesome"

The instant Haley touched the saddle, the mettlesome creature bounded from the earth with a sudden spring, that threw his master sprawling, some feet off, on the soft, dry turf.

1852, Harriet Beecher Stowe, chapter 6, in Uncle Tom's Cabin

Among them was the "West Country" driver, Swain, of Bude, over the Great Central, who reminded me irresistibly of a hunter putting a particularly mettlesome steed at the fences.

1948 September and October, Cecil J. Allen, “British Locomotive Practice and Performance”, in Railway Magazine, page 334