I don't know about wild boars. Like you say, they're probably not going to be sneaky, but videos do portray them as being pretty fast and tough, killing a couple of people a year. I wouldn't expect them to be quiet, but also not super loud either. But, I did a little searching and found comments from a person who has encountered them a number of times while hiking. They said wild boars were generally noisy in their normal routine and usually pretty easy to avoid, but they did get surprised by a pair of them once and was fortunate that they retreated. At this point, I don't know what to think. I've been okay with the TFP implementation.
Its mountain lions that I'd like to see worked on. Real mountain lions like to stalk and ambush. I wouldn't expect a mountain lion to make much noise. Rather than roam randomly around the snow biome I'd like to see them hide in the cover under the snowy pine trees and wait for a player (or zombie) to get near before they pounced. I wonder if that's some AI that could be codified.