Hold your horses on

This is another “maraphor”.  The speaker uttered this mix up, and then said, “I mean, keep your britches on.”  This is a congruent conflation of  “hold your horses” and “keep your britches (or pants) on”, both meaning to restrain yourself.  Britches, or breeches, are pants used in riding horses so I can see where the speaker was confused.  She probably visualized someone with breeches riding a horse.  A big shout out to Marianne Julian who heard this and passed it on!