Origin of the Goidels in North West Germany

[Taranis]

Ok, thank you. In this case it may have been a loanword.
It would have been a remarkable parrallel if Equus got dropped for Caballus and Ech got dropped for Capall without some influence of one on the other. Perhaps Capall came into the picture during the days of the Anglo-Normans.

Well, as I said, I'm not sure if it's a loan (but I suspect it). Also, from what I know it must be older. The peculiar part is that both the words "Capall" and "Ech" are attested in Old Irish. I'm just not sure how /b/ would yield /p/ (or even -VbV- yielding -VpV-), especially because it doesn't happen in other Irish words, as can be easily demonstrated.