One hippopotami
Cannot get on a bus
Because one hippopotami
Is two hippopotamus

Allan Sherman, to the tune of What Kind Of Fool Am I.
http://dmdb.org/lyrics/sherman.nut.html#8

Two elephant songs at
http://www.bbc.co.uk/schoolradio/pdf/wigglewaggle_summer_2008_programme05.pdf
The first one is similar but not identical to the spider web song that has been mentioned here.