Catchweight

They'll break from the gate in full tack with jockeys riding catchweight in silks.

Source: wiktionary

to ride catchweight

Source: wiktionary

There were also races for gentlemen-riders, in whch all the best performers of the day took part. Lord Glamis, afterwards Earl of Strathmore, rode his maiden race there—a catchweight match with Sir Watkin Wynn, a rare bit of fun got up by the officers of the 1st Life Guards.

Source: wiktionary

And— He reached the paddock and took up an unobtrusive position at the rail to watch the first race for which a mounted Police bugler had just sounded the “Boot and Saddle.” It was a catchweight sprint for local farmers' ponies.

Source: wiktionary