The court in the northwestern German city ruled against the white pony's owner, who wanted to tattoo the animal's right hind thigh to make it "more uniquely beautiful."
He had already shaved a large portion of hair from the animal and pre-tattooed the outline of the tongue, 15 square centimetres (six square inches) in area.
"The tattooing of a warm-blooded vertebrate contravenes animal protection laws," the court said. "This forbids causing an animal pain without reason."
In addition, the court took into account the inability of the animal to understand why it was being tattooed and said the owner's desire to "beautify" the pony masked a more commercial purpose.
"He wanted to make money from a tattoo service for animals," the court said.