I had a standardbred who could do that. Not that speed but she could boogie. I rarely let her open up but she would do it as her gear of choice. She would usually choose this gait at liberty in the pasture too when all the horses were messing around. Nothing harsh about it if you use your head. First ride of spring I would have a little trouble keeping her from racking too fast on the road. When we got to the dirt trails I would let her get it out of her system.
She was a track washout, a free legged pacer but rarely paced under saddle.
My TWH seems to have this gear too. I've seen her do it in the pasture but I haven't asked for it under saddle yet. She's kinda oddly green under saddle and I don't get to ride as much as I like so when I do we work on slow things.