Beasts refers to birds, insects, animals, essentially any natural fauna exists alongside Sentients and does not explicitly make an enemy out of them, nor whose purpose is solely to kill; this is aside from Monstrosities, which are unnatural and cannot live alongside Sentients in any capacity. The stats of many Beasts can be used in lieu of another that is not given, e.g. the stats for a lion can be used for a tiger or a panther, or slightly modifying the stats of a horse can be used to make a donkey, and so on.
If a character is attempting to handle a Beast and “persuade” it to do something for them, they must make a Beast Handling check against the Beast’s Insight check: on a success, they convince the Beast to trust them. If the character gives the Beast food or something it wants, they have advantage on the check, but if the Beast is angry or afraid of the character or if they are in combat with the beast, they have disadvantage.
On the whole, animals will generally only attack when threatened, frightened, extremely hungry, to defend their young, and when trained to. Otherwise, they will flee from danger.
There are 3 categories of Beasts, based on their relative size (different from their actual Size): normal, giant, and dire. Normal Beasts are those that are most frequently known and seen in the wild and are almost always Tier 0-1; giant Beasts are much larger than their normal variants and are usually Tier 1; dire Beasts are relatively massive, always Big or bigger (or in the case of the Dire Shark, much bigger), and are usually Tier 1-2.