Mackerel was originally an old French word, "maquerel" and when the Normans conquered the British in 1066, they brought maquerel into the English language.
The Oxford English Dictionary (OED) in 1989 listed 19 different mackerel spellings down through English-speaking history. When you consider that most people were illiterate at the time, there were no dictionaries, and no notion of "correct" spellings, it would have been highly unusual for only one spelling to predominate.
In any case, 19 is a lot different names to call a fish. Here's the complete list:
mackerel, macquerel, mackarell, makrall, macrill, mackril,macril, makarell, mackreel, makreill, maycril, maquerel, maccarel,mackrel, makrell, macrell, makral, mackerell, macrel, mackaral, makerell, macquerell, mackrell, makrel, makerelle, makerel, macrelle, makyrelle, makcaral, mackarel, mackeril
Mackril! Or Is It Macrell? NPR.