I heard this on the radio this morning but can't find any links to it.
Anyway, it seems a man wrote asking for a card from the Queen when his cat reachd her 16th birthday (100 in cat years, so he said). He said he sent it more as a joke and never expected to hear back.
Apparently the Queen did send a card and a note saying she understood his bond with puss and commenting on the comfort given by companion animals. So, don't know what to make of that - can't believe the Queen reads all her mail, so i suppose one of her staff thought she would actually like to reply to that one? Dunno