A good number of new pets do not accept their environment which they are going to spend into the foreseeable future. Some are too shy or frightened of their new owners and keep to themselves. This is very common with undomesticated pets though many domesticated pets exhibit the same inhibitions. They may even go to the extent of refusing to feed or be fed, grow weak and may eventually die. The problem is very common with herbivorous pets. Their carnivorous counterparts are more realistic in their approach and come to accept their environments more easily. On the other hand, some may become defensive and when stressed, they can attack albeit cautiously. Have you ever encountered such challenges?
When you say new pets, what kinds of pets were you referring to? Sounds like those certain kinds of animals are meant to be in the wild or perhaps these have been under so much trauma from the abuse of their former owners. I especially see these kinds of behavior from rescued animals. I tend to watch dogs being rescued on videos and I cannot help but feel really sad for them because they've been left abandoned. Some people are heartless and if they cannot show any kindness, they sometimes would hurt a dog on purpose.