If you own Poser, you should have it as "legacy content."  Even Poser Pro 2012 has the old Poser 4 horse included.  (You may have to change it from .crz to .cr2 format.)

If you do not own Poser, I don't think there's any way to get the Poser 4 horse.

Zygote was giving away the OBJ of the horse as free stuff, but it's just the OBJ.  It's not pose-able, and probably wouldn't work in the carousel.