You are correct, there are several reasons.

Is the horse current on a deworming program?
How old is the horse?
When was the last time its teeth were floated? THIS IS A BIG CAUSE FOR WEIGHT LOSS!
What is your horses exercise program consist of?

These are a few questions that need to be answered before we can go much further.

You could have an over load of parasites. It could be time to change up your horses feed depending on its age. It could be that the horse needs more calories due to an exercise program.

Give us a little history and answer the above questions and we can go from there.