According to the wiki:

Will attack things that are smaller than it (like dwarves). Only one group of "large predators" will appear on any given map (possibly two groups on "savage" maps). In adventure mode, large predators will try to ambush and attack you (and your party will attack them back). When tamed, large predators tend to be much more aggressive to enemies than non-large predators, making them a good choice for an animal army. They may go on rampages in worldgen, and adventurers may receive quests to kill them. Also, they can be mentioned in the intro paragraph when starting a fortress e.g. "ere the wolves get hungry."

A dwarf approaches a pack of beak dogs or a tiger to, say, fish in the nearby pond. Will they reliably attack him? I honestly haven't seen much of it, but that's because I usually keep my non-ambusher dwarves close to the vest.