|actually an edward lear illustration (PD)|
St. Bernard of Clairvaux, the great Cistercian abbot of twelfth-century France, is sometimes portrayed with a white dog; here's why. . .
However it be, he verified the Interpretation of his Mother's dream. She dreamed, when she was of Child with him, she should be delivered of a white Dog, whose barking should be very sonorous. Being frighted at this Dream, she consulted an honest Monk, who said to her, Be of good Courage; you will have a Son, who shall keep God's House, and bark much against the Enemies of the Faith. St Bernard did more, than the Prediction imported; for he barked sometimes against chimerical Enemies, against Errors, which were either meer Trifles, or unjust Interpretations of the Words and Thoughts of others...
-- From Bayle, P. (1734-38). The dictionary historical and critical of Mr. Peter Bayle. The 2d ed., carefully collated with the several editions of the original ... To which is prefixed, the life of the author, rev., corr., and enl., by Mr Des Maizeaux ... London: J.J. and P. Knapton [etc.]. p. 772-3.