‘The great pied hound with the belving tongue.’
Williamson uses this several times to describe baying hounds in pursuit of the valiant otter Tarka. According to the OED it’s an English regional word, mainly from the south-west, where Williamson wrote his book. It means to shout, roar, bellow or similar, and OED gives another example referring to the ‘belving of cows’.
See also ‘yarr’.
Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 16