I'm not sure how true the death of cavalry during WWI was in Russia. During the Civil War, traditionally tough units of cavalry troops called Cossacks fought for both the red and white armies. They were known as some of the toughest and most dangerous troops on both sides, so it makes sense that they would be featured in propaganda. On the other hand, they were also famous for committing atrocities, while fighting for both sides. As a result, it makes sense that the other side would feature enemy Cossacks in their propaganda, in an attempt to discredit the enemy by highlighting bad behavior.