What do i feed an underweight horse?

First blanketing. This will help them keep weight on, as they don't have to burn energy to produce heat. I don't think healthy adult horses that are in their prime should wear blankets, but foals and seniors definately should. The key is not to over blanket. I like to have a turnout sheet for when they are outside. Then in the barn and when its really cold, a stable sheet and fleece cooler or stable blanket should be underneath, or just those in the stall. This is most effective, because you can change layers depending on the weather. As for feeding, purina has a senior feed that many people reccomend. I don't know for the colt though. You can also give them some corn oil in their feed. I can't remember if its 1/4 cup or 1 cup, so you will have to look that up. And make sure to provide as much hay as they will eat.