Bruce, he was Steve's dog, had told Tony once that if he wishes for something very hard, it'd come true. So before he died he wished to become human, because he loved Steve so much, and as nice romantic fanfics go, the wish became true. Clint and Loki were cats also in this story (Clint was Natasha's and Loki Thor's), and later became humans. Bruce remained dog.
Yeah tha's about it. Since I couldn't find it again and no one could help me yet, I thought I imagined it or something, but, come on, I remember everything about it, so it must at least exist :D
Oh and in the end Tony even made a device so the humans could understand the animals as well.