Mallard was (presumably) an anthropomorphic duck.
Mallard was a rich 19th-century Duckburger with a flair for the dramatic. Before his death, he hid his fortune (referred to thereafter as "the Mallard Treasure") in a secret location, and concealed the map to it inside one of his andirons. He let it slip that the map was somewhere in Mallard Mansion and left future treasure-seekers figure out the rest. (Scrooge McDuck, having bought Mallard Mansion for the purpose of finding the Treasure, would later succeed in locating the correct andiron after much trials and tribulations.)
Behind the scenesEdit
Mallard is repeatedly mentioned, but never seen, in the 1976 story Key to the Treasure, which takes place after his death.
It is very possible that Mallard was an ancestor of Drake Mallard, an interpretation supported by the above fanmade rendition of the character (whose clothing side and poise are meant to evoke Drake's).