Muizeneus and Hondekop - mouse nose and dog head - are the nicknames given to the electric multiple unit sets from 1946 and 1954, known more officially as mat' 46 and mat' 54, i.e. matériel from 1946 and 1954 (see earlier post about the Blokkendoos from the 1920s).
The mat '46 sets were built by Allan, Beijnes and Werkspoor between 1949 and 1952 and used in 2-car and 4-car (2 x 2-car) compositions .
The mat '54 series were built by Allan in Rotterdam between 1956 and 1958. They were noted for their heavy weight per passenger (886 kg). Three versions existed, consisting of two, three or four coaches. To modernise the Amsterdam–Brussels service accompanying the finishing of the electrification between Roosendaal and Essen (respectively the Dutch and Belgium border stations), 12 EMUs were built to run in multiple unit with existing ones except that the special variant could run under 1500 V DC (Dutch system) and 3000 V DC (Belgium).
A hardcover book on them is still available (in Dutch). (These photos from fansites)