The current lore for Deino is in serious need of a remake, so it's kind of hard to look at right now! But, I'll still put the old version of it here until I replace it with a more updated version.

Deino is a demon from hell unaware of their origins. On this planet, (Yes, it's seperate from Earth) mysterious individuals are mutating other creatures (such as demons n' stuff) with other regular animals. Deino used to live with his parents, at least she thought she did. Until one day they found themselves in an unfamiliar place. He found himself behind glass and other creatures on the other side. She would usually find food inside the container he stayed in. They were still scared but got used to it, because they had to go through this for 3-4 months. Until one day, they found themselves in a large tube.( I don't actually know what it's called)) She felt like she couldn't breathe, the tube filled with green liquid. He was still able to look around, but was too afraid what was happening. He started banging against the glass with all these wires connected to her body. The people were going to mutate Deino with an animal, but Deino hadn't realized until she started feeling tingly inside her body. Soon he started feeling a shock of pain bolt through his body and he passed out.

Many, many hours passed and Deino woke up coughing violently on the floor. The experiment hadn't worked too well, Deino looked the same but with fluffier and more animal like proportions and instincts. Deino felt a lot taller than he used to be when he stood up. Suddenly she dropped back to the floor and threw a tantrum. Although He was mutated with the animal, he still had her demon powers within her. Not able to control what she was doing, Deino broke through the walls of that area. Ending up attacking other creatures without knowing. Her demon inside of her had taken control of her body but still was able to control his mind. Deino was fused with a goat, and realized Deino couldn't control his anger. His goat horns had moved in the front of his head and got spikier teeth. Along with longer claws and spikier/rough fur. After their tantrum was over they had a mental breakdown realizing what they had done. Also, Deino doesn't actually age, they became permanently an adult after getting fused. They still age on their birthday though. After Deino had a good idea of what caused that tantrum, he was too scared to go back home thinking they would hurt his loved ones. Deino was on the run. They've been hiding in the shadows and struggling to stay alive for 3 years. He also thought that those individuals were looking for him. Eventually he learned how to control his powers only a little bit better than she used to.

He still doesn't have the best memory of his old life but that'll forever stay a mystery.