Officers from Thames Valley Police were called to the 3,000-year-old chalk monument at about 2200 BST on Thursday.
They found the head and eye of the horse had been sprayed with purple paint. A banner that read “fathers 4 justice stop the secret family courts” was recovered from the scene.
New Fathers 4 Justice and Real Fathers for Justice both denied responsibility.
Police and members of the National Trust have been working to re-chalk the monument.
Sgt Steve Clark said: “The monument has been pretty much restored now and is back to how it should look.