Absolutely not. I would fear the horse that could.
It is illegal to deface public property.
If I had to guess, some poor soul received a citation for tying his horse up to a hydrant back in early 1900, damaging the wooden post supporting the water line.
The 1937 code prohibited: Fasten, hitch, or tie any horse or other animal to any fire hydrant, telegraph, telephone, electric light or other pole or to any fence, tree, shrub or other property.
Drive, stop, tie or place any horse, or other animal where it may bite, eat or in any way damage any fire hydrant, telegraph, telephone, electric light or other pole or any fence, tree, shrub or other property.