describes a shemale that is very very tall and/or heavyset (i.e. giant hands, large ass fake tits, adam's apple thats bigger than their balls, etc). Usually, a gendersaurus has an aggresive (and frequently) a bitchy manner.
real girl 1: *sees a gendersaurus walking down the street* AHHHHH!!!! LOOK OUT!!! ITS A GENDERBEAST!
real girl 2: hide ur boyfriends everyone!
little girl: mommy, im scared for life *gets really disturbed and confused*