Paul McJones has a large collection of old Lisp manuals, which shed a lot of light on historical questions like how unary
typep got its irregular name. The earliest reference I've found is in a 1973 Maclisp manual, which describes it thus, immediately after the other type predicates:
typepSUBR 1 arg
typepis a general type-predicate. It returns an atomic symbol describing the type of its argument, chosen from the list
(fixnum flonum bignum list symbol string random)
symbolmeans atomic symbol.
Randomis for all types that don't fit in any other category.
A “general type-predicate”? This wording suggests
typep got its name because it was seen as a generalization of the regular type predicates. This makes sense: as Maclisp acquired an increasingly unwieldy number of type predicates, its maintainers would want to merge them into one operation, and would probably think of that operation as “a general type-predicate”, even though it's not actually a predicate.
random type is probably just an implementation detail showing through, but it's still adorable. Rudimentary type systems are to languages what tiny clumsy paws are to kittens.)