Icabod Knox and Isadora "Izzy-I" Knox II are yellow sibling Knoxes who appeared in Dr. Seuss' ABC. Icabod is Izzy's older brother and Izzy is Icabod's younger little sister. While Icabod's name was mentioned in the book, his younger little sister Izzy was unnamed until the ABC software by Living Books.
They both adventure together to find what begins with each letter of the alphabet. Icabod's birthday is revealed to be in May. Icabod also has three costumes (a cowboy, a cook and a captain) that he keeps in his costume closet.
In Hop on Pop, there were also two other characters that looked very similar to Icabod and Izzy when they saw bees and fish in a tree.