The word someone is used for human beings but I am having a hard time understanding if it is also used for animals!
If you hear the word someone it almost certainly refers to a human being. Some people will use it fancifully to refer to a pet or other animal, but even then it will be ascribing human characteristics or personality traits to the animal.
For example, if the pet dog comes begging for food, the owner might say, "Well, looks like someone is hungry."
But if out of the blue you said, "Someone came to the door today," nobody would think you were talking about an animal.
Agreed with Robusto.
It could be used for (non-human) animals IF the user is implying that the animals have a "being" that is equivalent to humans. For example, in fantasy literature or New Age Spirituality.
"He had the feeling that someone was watching him." could mean a wolf in the forest, if the writer is ascribing some level of sentience to the wolf, for example.
However, such uses are very specific and outside the norm.
Re: the forest; If a human was the 'watcher' then the writer would use "someone" and this would communicate some form of danger or mystery.
If the writer wanted to suggest an animal, he/she would use "it" which leaves open the possibility of a threat (e.g. a predator) or mere curiosity (e.g. a rabbit or bird).