In our RMSS game, the healer had a wolf. I think the GM actually just used standard stats, but the wolf was controlled by the GM and the player could only try to persuade it to do things, with mixed success. It was more concerned about protecting her than about attacking the enemy.
The wolf and the mentalist did not get along at all. As far as he was concerned, it was a dangerous uncontrolled thing following the party around. And it could tell. Since he was afraid of it, it clearly "knew" that it was the alpha male of the party. Made for some interesting roleplaying moments. (The party fighter might have challenged it, but that just made her the alpha female as far as the wolf was concerned.)
There was also an amusing moment when it was sent to "fetch" my character. Not "find" or "lead us to" but "fetch"....
Taking it into towns was a bit of an issue. The player did not stick around long enough to get to an actual city, but it was an issue getting it onto a ship. Not as much as the issue of getting the fighter's warhorse on deck, though.
At one point we also had a ranger with a bear. The bear basically had to wait in the woods when we went to town.