Silent and terrified, the animals crept back into the barn. In a moment the dogs came bounding back. At first no one had been able to imagine where these creatures came from, but the problem was soon solved: they were the puppies whom Napoleon had taken away from their mothers and reared privately. Though not yet full-grown, they were huge dogs, and as fierce-looking as wolves. They kept close to Napoleon. It was noticed that they wagged their tails to him in the same way as the other dogs had been used to do to MR Jones.
I am wondering if the bold part is a passive form of a verb?
What is more, would you analyze that in a more readily way so that I could get it better?