Sorry, I don't know which story it is that you are looking for it does not sound familiar to me, but I also have a story that I was looking for…
I think it was on bdsmlibrary, but maybe it was on fictionmania(I doubt it..). They both have so many stories and equally bad searching that it is impossible to find anything…
There was multiple versions of it, I think it started out with a female on some and others had a male. The person wanders into a house or a barn and gets trapped in a room. There is usually a single object of some sort. Or just something in the room I remember one of them was pellets in a bowl. The person had to eat the pellets. Or a restraint like a mask or hood. The person would have to put on the object or eat from the bowl in order to progress through a doorway into the next room. The rooms would continually restrain and transform the person until they reached the exit. The exit would deposit the person into a stable or some fenced area with similar animals of the final transformation. I think one was a dog and another was probably a pony or cow or something. It was more of a latex/bodysuit transformation.