Not a patent, per se, since patents require publication. But intellectual property enforcement can exist outside of legal fiat through DRM, certifications, obfuscation (perhaps a special case of DRM), trade secrets, and trade organizations (like guilds). Not to mention social pressures for creators to respect the work of others (as in comics stealing jokes).
Point being, purely private intellectual property rights exist, but usually only to the degree that they are enforced by private organizations or the societies they belong to.
