"Last year’s Beaters, Derrick and Bole, have left, but it looks as though Montague’s replaced them with the usual gorillas, rather than anyone who can fly particularly well."
-- Angelina Johnson (OP19)
Regarding Derrick's physical attributes, Lee Jordan remarked that Slytherin captain Marcus Flint:
"....seems to be going for size rather than skill...." (PA15)
He was not considered to be very intelligent by Gryffindor captain Angelina Johnson:
"....I was always surprised Derrick and Bole managed to find their way on to the pitch without signposts." (OP19)
Derrick is a alternate spelling of the given name Derek, which itself is a derivative of the Germanic name Theodoric (meaning "ruler of the people") (Behind the Name). A derrick also a very large machine for moving heavy objects (an apt name for the typical Slytherin Beater). The surname Derrick was the name of a famous London hangman and came to mean a "hangman or gallows" (Oxford English Dictionary).
It is not specified whether Derrick is his first name or surname. However, as the British public school tradition is that all students are referred to by their surnames, it is likely that his given is the unknown.
Lexicon list of Beaters