Frankie and Benjy were Trillian's two pet mice who were actually pan-dimentional beings who created a super computer called Deep Thought to calculate the Ultimate Answer. Frankie and Bengy tried to remove Arthur's brain in order to discover the Ultimate Question. This because Arthur was the only piece of Earth left besides Trillian Astra, who left the Earth with Zaphod long before Arthur and before its destruction, leaving Arthur with the most complete copy of the Earth program in his brain.