I use Horses for followers mod, it works great, tried with Lydia. Now I made Ysolda my follower (she's a non-follower NPC in Whiterun) and she doesn't want to ride her horse. What can be the difference between Ysolda and Lydia? Anybody can tell me what to change?
I had this problem as well. What you usually need to do is go to a stablemaster and choose the "I want to buy a horse for my companion" option. Then, get on your horse and move a little bit aways, and the npc should get up on the horse as long as he/she is a follower. Sometimes the npc takes its' time thinking if it wants to mount, but should after a little bit.