are these poisonous for horses?

Photos?

Dwarf red-leaf plum could be Prunus cerasifera or Prunus cistena, I would think. Regardless Prunus are toxic to horses so to be safe I would assume yes unless specifically indicated otherwise.

Edited to add article on Prunus poisoning in horses:
http://www.equineguelph.ca/pdf...

This one explains a bit more about why horses are less affected than ruminants:
https://csuvth.colostate.edu/p...

Ahh, I thought it was a description and not an actual type of shrub.