Good fiction relies on endings that make sense. You don’t want to have your happy hug boxing story suddenly and unexpectedly end with the rescued foals getting ice picked by the up until now completely loving owner. See if you can guess how this story is going to end.
That said, having a frustrating ending is sometimes a great thing because fluffies almost never get what they want. The smarty lives another day because there is no justice, The mummah never finds food and slowly dies of starvation. The human got what they wanted from the fluffy and the fluffy gets nothing.
When you write, write with your ending already in mind. It makes the journey there easier.