MiltankRancher's opinion is really interesting, I hadn't thought to that. Because we don't know a lot of things about Keldeo yet, and will get the answers in Pokémon Gray (or in a movie?). At the beginning I was sure that the Pokémon wasn't a legendary, I hardly imagine Alder wandering with a Kyuurem or a Tornadus by his side... But Keldeo could do. And as we don't know a lot about it, we can guess that an event happened to it, and Alder thinks it's dead but actually it's not or something like that .