Answer:
If it's a "northern" forest, how about a wolverine? They are well enough known you can get good reference photos. They are not friendly but would be a great "pet" for a Dark elf.
For a "central" forest there is something called a fisher, kind of like a weasel on steroids. They are really nasty and creepy to watch move, but also very elegant. If you can't find a reference photo, just use a weasel and make it the height of a small dog, then keep the rest of its proportions the same as a weasel.
For a "tropical" forest, there are some really great big cats, a few really impressive snakes, and even some birds, parrots and others, that are beyond strange.