Let dogs delight to bark and bite.
For God hath made them so;
Let bears and lions growl and fight.
For 'tis their nature to. [ Isaac Watts ]

The canine letter - the letter R. A growling dog will bare his teeth and growl the letter R.

Society is infected with rude, cynical, restless, and frivolous persons who prey upon the rest, and whom no public opinion concentrated into good manners, forms accepted by the sense of all, can reach; the contradictors and railers at public and private tables, who are like terriers, who conceive it the duty of a dog of honor to growl at any passer-by, and do the honors of the house by barking him out of sight. [ Emerson ]