A quick Google search says pigs didn't gain weight from a diet heavy in coconut oil because it's almost purely saturated fat, and animals need a mixture of different types of fats in their diets. Specifically the coconut oil was too low in Linoleic acid, which is an essential Omega-6 fatty acid needed by animals to produce other fats in their body. Coconut oil is a fine source of calories in animal feed when part of a balanced diet including other fats.