Both seem to be native Gaelic words (first is cognate with equus, but not a borrowing from Latin, the second seems to be related to Latin caballus which might actually be a borrowing from Celtic).
I know of at least two other animals where Sc. and Irish took different routes naming them:
- dog: cù in Scotland vs madra or madadh in Irish (though cú exists in Irish for ‘hound’, madadh exists in Scottish; both have gadhar also referring to dogs – generally in Irish, apparently just to mastiffs in Scottish),
- bear: mathan in Scotland (the native word) vs béar in Irish (English borrowing), though mathúin (or mathghamhain in older spelling) exists in Irish literature.
Tha never means either I or you. It is a verb. It means is.
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