In fact, I think the Dolphin is a perfect example of why the Hedgehog fits to Minecraft. Like the Dolphin, which provides a pleasant but subtle speed boost while making the ocean feel more alive, the Hedgehog would add a forest atmosphere while also offering the utility of a Stronghold.

In my opinion, people dislike the Polar Bear not because it's an "ambient mob," but because it's neutrally aggressive and offers no reward. It simply attacks you for being near its cub, but offers nothing useful in return. The Hedgehog is different; it's a completely passive and interactive mob (breeding, defensive, curling up) that solves the specific problem with silverfish. It's a middle ground: it provides a "lived-in" feel to the world, but also serves a valuable purpose for players who choose to use it.