Lordinquisitor has it right. That had to have been Sandor on the Silent Isle. The monk spoke himself of how he was a Knight that did some regrettable things and 1 day he should have died but washed up on the Silent Isle shore instead and took a vow of silence for many years and became somebody new....then he happens to stumble upon the hound; now there's a really large gravedigger with a limp(Arya said his leg smelled infected when she left him), and he's really fond of dogs. Cleganes are hard to kill.
ADWD spoiler