Just a thought - and I don't get many
It 'may' also have something to do with the fact what we call it? eg we don't say "having a nice pig buttie" or "it's roast Cow for dinner"
Kind of seems a bit more 'unnatural' to say Horse as the image of Red Rum appears as oppose to the generic terms of beef pork and lamb for cow pig and sheep
anyway that was my thought
off for a lie down now - my head hurts!