Frankie and Benjy are the main antagonists in the first Hitchhiker's Guide novel, as well as its 1981 TV adaptation and its 2005 Disney/Touchstone film adaptation. They seem to be small harmless mice, but are actually highly intelligent, evil Magratheans with the intent to cancel the experiment to find the Ultimate Question.
In both the radio series, Frankie was voiced by Peter Hawkins while being portrayed by Stephen Moore in the TV adaptation and Benjy was both voiced and portrayed by David Tate in both the radio series and TV series. In the 2005 film, Frankie was voiced by Garth Jennings, and Benjy was voiced by Zoe Kubaisi.
They came to Earth after it was made by their computer Deep Thought and they disguised themselves as mice, (which is actually possibly the closest humans could see their true forms) and they played a psychological game on humanity.
Their true forms are tall humans, about seven foot, and they are rather pale. Frankie and Benjy were revealed to be in league with the Vogons, because they want to stop everybody else finding out the Answer and want only the very rich to know. They tell Arthur they need to cut open his brain and thus kill him in order to know the Question, because he is human and survived Earth being blown up.
However, because they are mice at the time, they have limited power and agility, and as such are easily escaped from by Arthur and his friends, and eventually come up with a new solution - to fool Management into thinking they have the Question when they don't.
In the movie they are vastly different, instead of being seven foot tall humanoid males they are short female children humanoids with purely black eyes. They're backstory remains the same however although their role in the climax is more sinister as they drug Zaphod, Ford and Trillian to fall asleep and cuff Arthur to a seat and actually almost succeed in removing his brain before the latter kills them with a tea-pot, making them fade out as opposed to just running away from them.