Up here in Ontario, it's really common for horse riding stables to feed beet pulp, but the issue with it is that it's just a filler, and really does nothing more than fill the horse up. It's used in cheap dog food as well, and it's not THAT great for putting weight on a thin animal. Really, you should go with a high fat high fibre food for a thin animal, we fed high fat high fibre to our horse after he lost a buttload of weight because of his heaves. He gained the weight moderately (not TOO fast, but fast enough) and kept it on, it also gave him the boost he needed to get healthy again.