Username: My Horse Rocks
Name: Fire
Gif about them:

Why are they so radioactive?: (249 words) Fire darted in and out of the reeds, he swam up to the side of the pier and looked up. He had decided to find out what was going on: there had been loud noises for a while. Flipping his tail, he splashed out of the water and did a summersault in the air. As he spun, he saw huge, metal monsters, which were crashing about and pouring some kind of neon stuff on the ground. It pooled around, and almost spilt onto Fire, but he dived back into the water just in time. Peering up again, he fell backwards into the water again as a large pot of the neon stuff started to tilt towards him!
"Help!" he gasped as it started to pour on top of him. "Ow! Ow, it's hot!"
The gooey liquid burnt his fur, and hurt, oh, how it hurt. Fire's fur started to fizz, and it bubbled. Some little green and bright yellow bubbles popped up over his fur. Terrified, Fire swam down, down, down, until it was so dark, he'd normally never be able to see. But, and this surprised him, the stuff on his fur lit up the area around him. He noticed it was settling onto his fur, almost as though it had almost been there. Although he tried to rub it off on a rock, it just stayed on.
About two days later, it still hadn't come off. However, he was starting to like it!
And so that's how Fire got so radioactive.
What happened to their tail?: (100 words) Although Fire loved his glowing fur, others didn't, including humans, who thought it weird. A human saw him playing in the water and thought they could capture him to show off as a freak.
So, they lowered down a net. For many days they did this. Fire managed not to get caught, but then one day the net caught around Fire's tail and pulled tight. Although Fire struggled, that made the net tighter. It wrapped around and Fire realised his tail would have to break before he would be free. He eventually bit it hard, and it tore.
(That took so much editing to get it down to 100 words xD)
Extra: I really like this one!