"Woof," Gaspode said.
Peanuts
For posts about the comic Peanuts, by Charles M. Schulz
Is this the only instance of Snoopy speaking English?
Looks like not the only instance, but it is very rare. Here's another one:
And from the trivia on the wiki page:
Despite being a dog, Snoopy is apparently capable of communicating words other than "woof" and even being understood by the other Peanuts characters outside of writing his thoughts on his typewriter. One such instance is when his brother Spike was introduced: Snoopy heralds Spike's entrance in the August 13, 1975 strip by declaring in a thought bubble Spike should be served Eggs Benedict, but Lucy responds that the order should be changed to "ten pounds of buffalo steak", leading the reader to infer she somehow heard him. In a more explicit example, in the December 23, 1989 strip, Snoopy vocally says to Sally (so indicated by a speech bubble instead of the usual thought bubble) "Who cares? Merry Christmas, sweetie! Woof woof woof!"