The Stone Ixupi is an unseen Ixupi, since it was encountered in events prior to the game. One can only speculate on it's appearance.
Possible hiding spots
Again, one can only speculate on the hiding spots of the Ixupi. One such hiding spot is the Stone Buddha in the Gods and Religious Items room. Due to the notoriety of Buddha, anyone is prone to stop for a few moments, and gaze at it. There is also a stone mask in the Shaman's hut in the Shaman's exhibit room. It is a perfect hiding spot for when the Stone Ixupi for if someone is coming after it with the appropriate vessel, since it is off the main path.
Escape of the Ixupi
It can be figured that the Stone Ixupi was the third of the thirteen Ixupi to escape from the museum. The Ixupi escaped when Professor Windlenot attempted to flee from the museum. It is very possible the Professor might have thought the cave underneath the museum was safe, since the Ixupi are restricted to the museum grounds. It is possible that it attacked when he was trying to leave a warning for anyone who found their way into the museum, since he was scratching the symbol for their exhibit in the ground. Since an underground cave is entirely stone, it was a perfect trap for anyone to walk into. The stone Ixupi had drained
Professor Windlenot's essence right then and there.
The Stone Ixupi's vessel is the first one found, since it is right at the entrance to the tunnel that leads to the elevator. Once it is opened, the spirit of Sir Hubert Windlenot, whose corpse is on the other side of the lake, is released. Once the spirit is released, the spirit gives valuable advice regarding the Ixupi.
Being one of the three vessels to be found intact, it's complete form is seen right away. It is a round container, which has head that looks like a cross between a chicken and a lizard. The container also has a handle on it, and the pictogram for stone. The opening for the vessel is a round hole, with the bottom of it looking like teeth.
Loose in the world
Given the vastness of the world, odds of encountering the Stone Ixupi would be rare. Unfortunately, good hiding places for the Stone Ixupi might be quarries, mines, and canyons. One could speculate that the Ixupi could also hide in concrete. It may be aware that the vessel containing it could be found and used again to capture it, which would keep it from attacking a heavily populated area.