They don't believe you.

I was only 15 years old when it happened, and I have to say, it was the most terrifying experience of my life. My parents and sister had just dropped me off at home after a game and left to run some errands. With a quick goodbye, I went inside to watch some TV. After throwing my heavy duffel bag containing my uniform and shoes on the floor, I grabbed a pear from the kitchen and sat on the couch, turning on Breaking Bad on Netflix. My team had lost, but it didn't really matter. It wasn't an important game and the rest of the team and I were determined to win the next one. But, for now, we just had to keep practicing.
After about a half hour or so, I heard a soft but obvious thud from upstairs followed by uneven footsteps, as if someone were limping. My dog, Benny, a 2-year-old beagle who had been tied to a metal rod sticking out of the ground outside, had started viciously barking.
Confused, I pushed the situation aside and figured my sister, Kate, had just dropped something on her foot or something. But then I remembered that Kate was off running errands with mom and dad.
Benny just wouldn't stop barking. It was starting to get really annoying, and after a while, I couldn't take it anymore. I opened the back door and called him in, hoping he would forget about whatever it was he was barking at and shut up. But, strangely, he ignored me and kept barking, staring straight up at the roof. Fed up and frustrated, I slipped on some sandals, walked outside, and grabbed Benny by the collar. Now, having noticed that the door was wide open, Benny dashed towards the house, knocking me over in the process. Groaning, I got up. "Dumb dog," I muttered, wiping dirt and grass from my pants. But, when I turned around to go back inside, I noticed something strange. My bedroom window was wide open. I never remembered opening it and didn't think there was a reason for Kate to open it.
Pushing the suspicions aside, I decided I would go upstairs and close it before more cold air made its way inside the house. Fortunately for me, Benny was fianally done fussing by then and was taking a drink of water.
I went up the stairs and opened my bedroom door, but what she saw was just impossible. A hideous, human-like monster with black, gaping eyes, crispy flesh that looked as if it had been torched, long arms and sharp, razor-like fingers that dragged on the floor, and no clothes was sitting on my bed, staring at me. I stumbled back, too terrified to even scream as I looked back at the horrible beast. It just stared at me with its pitch-black eyes, staring at me, not doing anything.
I quickly slammed my bedroom door and ran downstairs into the basement, putting a chair against the basement door at the bottom of the stairs so it wouldn't be able to get in, or, at least, I hoped.
Crying quietly but hard enough that it was difficult to breathe, I sat in the corner of the unfinished basement, praying that the beast wouldn't come down. I could hear those same uneven footsteps upstairs and an occasional growl, similar to the squeal of a baby pig that was being tortured, only very low-pitched, which only made me cry even harder. "I'm dreaming," I told myself. "This is just a dream."
But I knew it wasn't a dream. It felt too real. It was all too horrifyingly real. I tried to man up and quit the crying, but even so, a terrified stream of tears flowed down my cheeks and dripped from my chin.
At one point, I even heard my dog yelp. I was absolutely terrified and shaking violently, waiting anxiously for my family to come home.
Finally, after what seemed like a whole year, I heard the garage door open. I shoved the chair out of the way, opened the door, and ran upstairs faster than I had ever ran before. I grabbed my mom by the shirt and practically dragged her upstairs. All the while, she was shouting at me and demanding an explanation. I was panting and sweating, dry tears still sticking to my face. "Mom," I said through pants once we got to the top of the stairs, "you're not gonna believe this, but I saw something in my room. I think it's still there."
Before my mom even had the slightest chance to argue, I grabbed her again by the arm and took her into my room. But when I opened the door, the beast was gone. The window was still open, but the monster wasn't there anymore. It wasn't under the bed, either. Confused and frightened, it still had not occurred to me that the terrible beast could have left or been hiding in another room in the house.
After a quick talking-to, my mom, who apparently was having a bad day, went back downstairs and started preparing dinner.
Frustrated and sweaty, I finally closed the bedroom door and went downstairs to eat. Throughout the meal, I couldn't stop thinking about the monster an its terrible black eye sockets. How they stared into my soul, forcing waves of fear and panic into my chest. I remembered how hard it was to breathe and how dizzy I was but still somehow managed to make my way into the basement without passing out.
After dinner, I went back upstairs into my bedroom and turned on the light. To my realization, I saw that monster was back, breathing heavily and sitting on my bed just like before. I saw in horror that the corpse of my dog was lying on the floor, his neck slit open and his head skinned to the bone, except for his eyeballs, which hung out of their sockets, connected to the head only by thin strands of muscle. Written on one of the bedroom walls in Benny's blood were the words "They don't believe you."
Terrified, I let out a blood-curling scream and ran down into the living room where my family was now watching TV, crying. My parents asked her what was wrong, but no matter how hard I tried, couldn't speak. I choked on her own words as she tried to tell them about the monster, and Benny, and the blood...
I had finally calmed down when I saw my dog running downstairs, his floppy tongue hanging out of his mouth, leaving a trail of slobber behind him. I was astonished. Had I only imagined his body laying on the floor? No, it was all too real.
My and I parents went back upstairs into the bedroom. The blood and the monster were gone. The window outside was closed.
Those words will haunt my dreams forever. I'll never forget the monster, or the corpse of my beloved pet, or the way it felt like all my life energy was being drained from my body. Now, every night I can always see the image of the beast crouching over my dog's body, the words "they don't believe you" written on the wall behind it.