From Chapter Four of Frederica, where Alverstoke is first meeting Jessamy, Felix, Charis and the dog Lufra. Frederica explains to Alverstoke the origin of the dog's name, which is from The Lady of the Lake by Sir Walter Scott.
Lufra was the heroic dog who protected Douglas in The Lady of the Lake; in the Merriville's own life their dog had saved Jessamy from a rampaging young bull, so the girls then named him Lufra.
"the Monarch bade let loose a gallant stag.
And Lufra--whom from Douglas side Nor bribe nor threat could e'er divide,
The fleetest hound in all the North, brave Lufra saw, and darted forth.
She left the royal hounds midway, And dashing on the antler'd prey,
Sunk her sharp muzzle in his flank, And deep the flowing life-blood drank!"