The Halloween Hare is a creature that appears on Halloween on October 31st every year in Rookgaard, Dawnport, and Mainland. I'd classify The Halloween Hare as an event creature, and maybe not a boss. He doesn't drop any loot, you can't damage him and he can't damage you. He just is there for fun during Tibia's Halloween event and turns you into a creature as a costume. He can turn you into a Vampire, Skeleton, Pumpkinhead, Ghost, Witch, or Mummy for 100 minutes (the timer is only when you're logged in). To know where he is I recommend you click here to know where to find him. Just know he wanders around and can be lured, so you might need to check all over the general areas. I also wanted to include some parts of a book that describes the Halloween Hare's creation:
"...Humans had grown accustomed to be afraid of the dark and the unknown. This weakened their resolve and with that also the forces of good. The gods came together to find a way to rise the spirit of humanity once again. They discussed and debated but could not come to an agreement because their views were too contrasting. And while they still deliberated, suddenly one of their least and minor creations stood amongst them. It was the lord of the rabbit race who had passed unnoticed by the mighty guardians. Without fear the little rabbit faced the gods who looked upon him in astonishment. Then the lord of the rabbits proposed his idea to the mighty and great gods. Since rabbits are tiny and their enemies are many, they are quite familiar with fear and how to handle it. So the lord of the rabbits came up with the idea to have the humans overcome their fear by learning to laugh in the face of evil. The gods were amazed by this simple but clever idea. So they chose the mightiest warrior of the rabbit race and bestowed magical powers upon him. The daring hare gained the protection of the gods and the power to disguise humans with hideous appearances of monsters. Each year, the hare would appear for some time and do his work. So the humans learnt to laugh and had fun while appearing as monsters, and the great fear that had darkened the souls of humans gradually vanished.
This enraged the forces of evil and so they called forth the most malevolent spirit from the realm of darkness. It was his task to spread fear among mankind once again. The spirit was sent to take possession of the greatest living hero among the humans and to wreak havoc in the name of darkness. Unbeknownst to them, the lord of the rabbits had anticipated that the forces of evil would strike back. So he had sent some spies to the realm of darkness and learnt about their plan in time. He warned the humans about the approaching danger and once again had the right plan to save the day. He asked the humans to build a puppet clad in a shining armor and to put it on a throne in the midst of their festivities. As the puppet's head served a pumpkin with a carved face. The clever hare had figured that a spirit from the depths of the realm of darkness was not familiar with humans and would assume the figure on a throne in the centre of festivities as humanity's greatest hero. And indeed he was proven right when the spirit arrived and tried to possess what he assumed a human body. But the spirit had immense powers and so he finally managed to take possession of the pumpkin head on the cost of nearly all of his powers. With hardly any power left, the spirit made the pumpkin move and began to attack the celebrating humans. But the humans had long regained their courage and were not afraid to fight back. They drove off the evil pumpkin that escaped to some remote place where he was finally banished from the world. It took the spirit one year to regain enough of his strength to fight his way back into reality. He re-entered the world at the place of his defeat and to his shock and dismay he was no longer a spirit but bound to a pumpkin-like body. The beast roared in rage but could do little about it. Again he tried to wreak havoc in the world but again valiant heroes stopped his rampage and banished him for another year. Since this day on, the spirit returns every year hoping that one day humanity will be weakened enough for him to finally take revenge." Source: tibia.fandom.com/wiki/The_Rise_of_the_Pumpkin_(Book)