If animals could speak as fabulists have feigned, the dog would be a blunt, blundering, outspoken, honest fellow, but the cat would have the rare talent of never saying a word too much.

Philip Gilbert Hamerton (1834-1894) British artist, art critic and author.

Chapters on Animals, ch. 4 “Cats” (1877)
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A variant of this shows up in Agnes Repplier, The Cat (1912):

If animals could speak, the dog would be a blunt, blundering, outspoken, honest fellow; but the cat would have the rare grace of never saying a word too much.

This in turn seems to have been further reduced and misattributed to Mark Twain:

If animals could speak, the dog would be a blundering outspoken fellow; but the cat would have the rare grace of never saying a word too much.