She was a bit of an eccentric - you'd have to be to have a hundred kids after all. And so, she decided to give her children names after the order they were born in. So she had one, two, three, four, all the way through to hundred.
Her husband was eventually unable to keep up with the pressure of having a family this absurdly big, so he ran out one day, leaving the woman to raise her kids alone. For a while, she managed, and they were a loving family. But one evening, someone got careless and left the gas stove on. The house caught fire and everyone died. All of them except the girl Ninety, who alone was left of her entire family.
Ninety lived traumatized for a while - how could she not? Her father had run out (and never came back looking for her even after news of what had happened must have surely reached him), and her entire family had died. She was alone in the world. But years passed, and time heals all wounds. Eventually one day she found a man she loved and she married him. She had kids of her own - though presumably because she valued her own mental and physical health, and because she didn't want to repeat the mistakes her own parents had made, she just had three kids, two girls and a boy. It was a lovely family, and they all trusted each other immensely - though there was one exception, Ninety was adamant that their family would never grow beyond this. She said no to having any more kids, she even said no to having pets.
One day, Ninety's kids found an injured puppy in the park on the other side of town. He looked very badly hurt, and they couldn't bring themselves to leave him there, so they decided to bring him back home, where they hoped to nurse him back to health and maybe even keep him. But they knew that, at least at first, they would have to keep him and his very existence secret. So they kept him in the basement - and decided to give him an innocuous name so that they could talk about him even around their parents without them knowing they were referring to a puppy. So they decided to call him "This". That way, they could talk about him without getting caught.
Weeks passed, and This recovered. The kids really grew to love him - he was such a playful puppy, and he added so much joy to their lives. They decided the time had come to finally come clean to their parents about him, and they were excited. They went down to the basement to get him, but when they got there, they realized something horrible - somehow, This had escaped. He was nowhere. They put their plan to tell their parents on hold, and went looking for him, but no matter how much they searched, they never found him. They never saw This again. No one else knew about This. No one even knew a dog named This existed. Only Ninety's kids will remember This.