Aww, that's cute! Cute and a violation of copyright, per Character Arts, the company that represents licensing concerns for the Rudolph character (officially owned by the cleverly-named Rudolph, LLC). You can get the full story over at Boston Sports Then And Now, but the gist is that the Red Sox proposed the sale of a book for charity in which David Ortiz helps Rudolph vaguely "save Christmas." The deal fell through because Rudolph's owners "safeguard its current mythological reverence."
So the Red Sox went and appropriated Rudolph anyway, for their online holiday greeting card (the story changed, this time depicting Rudy as a minor-leaguer teased about never being able to make it to The Show). Alas, that appears to have disappeared from the internet, as links to the card are now re-routed to the Red Sox home page.
Looks like baseball isn't one of those reindeer games.