There may be design guidelines in the covenants or the covenants may permit the board or design committee to implement design guidelines. Depending on what the covenants say, the board or the design committee may already have the power to address these concerns in the future. If the stable has already been approved by the board or design committee, it will be more difficult to revoke the permission. You may be stuck with the placement of the stable at least until the stable needs to be rebuilt. Many covenants also have restrictions on the type of activity that may occur in the project.
Many residential projects have restrictions against commercial activity or breeding of livestock.
Finally, the complaining neighbor may have a claim for nuisance. He or she may be able to bring an action against the stable owner for creating or maintaining a nuisance on an adjacent property. This claim would depend on many facts not contained in your question and would be between the two owners. The association would typically not be a party in the nuisance action. He concentrates his practice in the area of homeowner association law.