who are skeet and nig

"Buck makes friends with John Thornton's other dogs, Skeet and Nig, as they await the raft that will carry them to Dawson. Skeet is motherly and nurses Buck's wounds during his convalescence. Nig is also quite friendly. The dogs manifest no jealousy towards Buck. He is welcomed into their frolicking games. The kindness of John Thornton arises from them as well."