About 10 or 15% have long lyre-shaped horns. It depends what you define as rare. I wouldn't call that rare. I'd call it uncommon.
Wouldn't it have to be maybe 5% or less to be rare?
There are some pretty strange ones, like these long-horned, long-necked Aries/Taurus from Walters 734 or Ludwig XII 8. Many people draw sheep, goats, and bulls like they are horses. I saw one manuscript full of beasts that all looked like dogs (they were labeled as many different animals but they were all basically dogs):
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Wouldn't it have to be maybe 5% or less to be rare?
There are some pretty strange ones, like these long-horned, long-necked Aries/Taurus from Walters 734 or Ludwig XII 8. Many people draw sheep, goats, and bulls like they are horses. I saw one manuscript full of beasts that all looked like dogs (they were labeled as many different animals but they were all basically dogs):

