Beta Dalek was a Dalek designation.
Following the Dalek-Movellan War, the Beta Dalek served the Supreme Dalek and was part of the attempt to rescue Davros from the prison station in 4590. (PROSE: Resurrection of the Daleks)
Another Beta Dalek accompanied an Alpha Dalek to Necros to capture Davros for trial. (PROSE: Revelation of the Daleks)