Horse Happiness

I bought three horses from the Rescue Center and their happiness is really bad. Does this mean they're going to pass on or is there something I can do to bring it up?

by not keeping your horses happy or caring for them will result in your horses dying

Give them treats and feed/water them and make sure all their care is up to date. It will increase their happiness daily (as long as you care for them daily) until it gets to 100%. Then you just need to maintain it. 

As to how the horses you got from the RC with low happiness happened, some people don't care for their horses daily and it lowers the happiness day by day. Then they decided to just put them in the RC more than likely.