There are two ways to lose one's spirit animal.
1. Your spirit animal is taken
2. Your spirit animal is killed
Losing your spirit animal isn't like losing a pet. Once your spirit animal finds you they become a part of you-they latch onto your soul. They think and feel and breathe with you. They share your life, they share your love, they share your pain. Losing your spirit animal is like losing your humanity. Once you lose your spirit animal you will never regain that piece of yourself. It will be lost forever.
Once every witch and wizard turns 15 their spirit animal senses them. No matter where in the world they are, they will feel you and they will cross countries to get to you. Some people never find their spirit animals, some don't find them until they are into their adulthood, but most find them in their teenaged years. In some cases, extreme cases, your spirit animal may appear beside you in a flash of magic. This can only happen if your life is in danger. Your spirit animal will always protect you until the day you both die.
In some cases though, one is left without the other. Once one finds their spirit animal there is no going back. People can live their whole lives without their spirit animal, but once the connection is made it is unbreakable. Even in death, you are connected to it. That is what makes losing your spirit animal so horrible, so earth shattering. Every day you walk the earth, you feel as if a part of you has died, and in a horrific sense, a part of you has.
People who lose their spirit animals lose themselves. They become angry, withdrawn, bitter and, in some cases, violent.
By this point you can probably guess what I'm about to tell you regarding a certain wizard-one who left his life behind and turned his back on humanity.
"James! James wait up!"
Peter puffed as he ran around the corner, chasing his friend. Well, friend was a strong word. James just sort of let him tag along. Peter was okay with it, though, because James was cool.
He heard James' laughter coming from the trophy room, the sound echoing around the vast cavern.
"Hurry up, why don't you!" James shouted back, stopped in front of the largest cabinet he could find. Peter came to a halt a couple paces behind him. He watched as James drew his wand, his eyes going wide and a small whimper slipping past his lips.
"James... this is a bad idea, James," he whined, covering his eyes with his hands and shaking his head. If they got in trouble, if Filtch caught them, then they would be hanging by their toes in the dungeon. Peter had heard the screaming, he never wanted to be put in that position ever.
James rolled his eyes and smacked Peter lightly on the back of the head. "Don't be such a baby, I'm just going to lift it off the ground a little. You saw me in Charms class, I'm the best one in there."
Peter didn't look convinced. "No you're not, Remus is. You couldn't even lift the feather. When you cast the spell you just ended up throwing it across the room. You can't do that in here, you'll get us in trouble!"
James sighed loudly and ignored him, steadying his eyes on the cabinet, focusing hard before flicking his wand. 'Swish and Flick'.
Peter saw her right as James started to speak the words.
"James, no!" he screamed, putting his hands out to try and stop the 11 year old boy, but it was too late. Peter's shout had startled James, causing him to cast the spell wrong. Instead of the cabinet lifting off the ground it exploded in a burst of wood and glass. James and Peter were knocked to the ground, their arms over their heads as the glass rained down on them. The sound of the explosion was mingled with a blood curdling scream.
It took a few moments before either of them lifted their heads.
"Filtch definitely heard that," James said, his voice quiet, but Peter wasn't listening. He got to his feet and walked slowly over to the remains of the destroyed cabinet, the glass crunching under his shoes. He stopped in front of the heap of wood and distorted trophies, his eyes wide as he stared down at the long python.
The snake had slithered into the rooms and under the cabinet right before James had cast the spell. It had been directly in the line of fire. Its remains lay in the wreckage, the floor splattered red.
"James," Peter breathed, his heart in his throat. "James, she's his. That man. The one who came to the castle today to look for Professor Dumbledore. Oh gods, James... she was his spirit animal. James, you killed his spirit animal!"
James grabbed Peter's arm, yanking his eyes away from the scene as he turned the young boy around and stared straight into his eyes.
"Never speak of this," he ordered, his voice hard and leaving no room for question. "Do you hear me, Peter? Never again. This never happened. Forget about it. Swear to me you will never mention this to anyone."
Peter stares into the eyes of his friend, his own eyes so wide they threatened to pop out of his skull.
"I promise," he whispered. "I swear... it will be our little secret, right?"
James nodded slowly, letting out a soft sigh of relief.
They heard footfalls seconds later-someone was running towards them and it didn't sound like the mismatched feet of Filtch that they were almost wishing to hear right now. Anything was better than what it was. They knew who it was, they knew it was that man-the one who had called himself Lord Voldemort but Dumbledore had called Tom-and they knew that if he found them he would kill them.
"We have to go," James said in a panic, grabbing Peter's hand and yanking him out of the room, running down the corridor as fast as his legs could carry him. "Don't look back, Peter, keep running!"
They ran all the way to the common room and didn't stop running until they were both tucked under the blankets in James' bed, panting hard and still clasping each other's hands, both shaking and both terrified.
"Our little secret?" James asked, his voice shaking and tears sliding down his cheek.
Peter nodded, his own tears matching the boy in front of him. "Our little secret." He squeezed James' hand.
There was a reason James always defended Peter when Sirius picked on him. He did his fair share of teasing as well, but he would always protect Peter because Peter shared something with him-something that Sirius couldn't even begin to imagine.
There was a reason James chose Peter to be secret keeper instead of Sirius or Remus.
Peter had kept his secret this long, he trusted him enough to let him keep another.
It wasn't just chance that lead Tom Riddle to Harry that night. It wasn't just a choice between two little boys. Harry wasn't just unlucky.
Voldemort tortured Peter. He tortured him and threatened him for information on how to get past the wards. But that wasn't all Peter gave him. After he got the information he needed he tortured Peter more, just for the fun of it. So Peter told him everything. Peter told him their little secret.
Voldemort went there that night to exact revenge on James, not only to kill Harry and alter the prophecy. James had ripped the little humanity he had left from him, leaving him an empty shell wanting nothing more than to kill, to destroy and take over. Killing James had let him feel the first sense of true happiness he'd felt since Nagini had died.
Years later, when he found a Python in the woods after taking Quirrel as his vessel, he felt love. He missed his Nagini, so he took her as his own-he made her into a Horcrux, the closest he could get to remaking his spirit animal, and named her Nagini after his lost love. She is and forever will be the closest thing Tom Riddle has ever had to true love.