Loop keywords are not true keywords; they are special symbols, recognized by name rather than object identity, that are meaningful only to the loop facility. A loop keyword is a symbol but is recognized by its name (not its identity), regardless of the packages in which it is accessible.
In general, loop keywords are not external symbols of the COMMON-LISP package, except in the coincidental situation that a symbol with the same name as a loop keyword was needed for some other purpose in Common Lisp. For example, there is a symbol in the COMMON-LISP package whose name is "UNLESS" but not one whose name is "UNTIL".
If no loop keywords are supplied in a loop form, the Loop Facility executes the loop body repeatedly; see Section 18.104.22.168.1 (Simple Loop).