Lyrre gives  a ponder on the nature of demons and the planes.   The idea of finding a spell caster to summon up the weakest demon they could manage is tempting, but the nature of Summon Monster spells will keep the party from holding onto the tail, given that the spell only conjures up an astral projection of the creature rather than their actual body, so the tail will vanish along with the creature when the spell's time is up.   The party will have to find a demon that is physically on the Material Plane (the plane you're on).  Hopefully there's weaker ones wandering about.