The Oxford Names Companion, OUP "Scottish Forenames" - Donald Whyte, FGH, FSG
Personal communication
Notes:
English pet name for Richard, from Old German "hard/strong ruler", and for Frederick, from Old German "peace ruler". Also may be a pet name for other names ending with the element -ric(k) (see examples above).
Thanks to Donald Whyte who provided much of the initial information for this site.