Covenants can be enforced by other than an HOA. The entity that put the covenants in place can enforce them for sure. Others would have to prove they have standing to sue to enforce them.if there isn't a HOA, nobody is able to enforce them other than the majority of the residents of the subdivision
Case in point, a buddy bought land from a farmer to build a house on, the farmer put covenants on the property. The farmers covenants prohibited any pigs being raised on the land and limited the number of horses and cows. It also prohibited buildings close to the property line greater than the county regulations.
That farmer and anyone who subsequently buys the farm has every right to enforce those covenants…