The Common Shelduck looks like a small short-necked goose in size and shape.
Sexes are similar, but the female is smaller. 
Both have a red bill but the male's bill has a prominent knob at the forehead.

1st, 2nd & 3rd photo: A male and a female. Waverveen, The Netherlands, 28 March 2017
4th photo: A male. Durgerdam, The Netherlands, 21 March 2017
5th photo: A juvenile. Anna Paulowna, The Netherlands, 1 August 2018