Fern Walters, from Arthur
Fern Arable, from Charlotte's Web
Fern, the GreenFairy from the "Rainbow Fairies" book series
Fern Test - Laboratory test usually used to determine if there has been an amniotic fluid membrane rupture during pregnancy, if positive a very pretty 'fern' crystallization pattern appears on a microscope slide (http://en.wikipedia.org/wiki/Fern_test)
Fern, the plant
Fern, character from episode of "Sex and The City"