I don't know any ballads or legends about this kind of being, but I confirm that European present and past folk literatures are full of lore about nickers. I would dare to say that the hippokampoi and ketea of Greek mythology - i.e. the Nereids' water horses - belong to the same family of beings.
I would like to trace a map of these fantastic creatures - proven there already doesn't exist one.
Online and printed dictionaries seem not to be very helpful, as they give sheer definitions and very small hints.
Can anyone suggest me any reading in English or German where I can find something more about Nickers/kelpies/etc.?