Ornithorhynchus maximus was a platypus from the Late Pliocene to Late Pleistocene. Some scientists believe it is an aquatic version of an echidna whereas others still believe that it is a platypus.
References
Hall, B. K., 1999, "The Paradoxical Platypus", BioScience, Vol. 49, No. 3 (Mar., 1999), pp. 211-218