Beasts refers to birds, insects, animals, essentially any natural flora or fauna exists alongside Sentients and does not explicitly make an enemy out of them, nor whose purpose is solely to kill; this is opposed to Monstrosities, which are unnatural and are expressly inclined to kill Sentients and sentient life. 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 an arbitrary DR set by the GM: on a (Critical) Success, the Beast trusts them and will willingly play along/obey until given a reason not to; on a Partial Success, the Beast is wary of them but will not attack them, and may hesitantly play along/obey; on a Failure, the Beast does not obey them; and on a Critical Failure, the Beast distrusts them completely and may even attack 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 they have disadvantage - if they are in combat with the beast, they have double disadvantage.
On the whole, animals will generally only attack when threatened, frightened, starving, to defend their young, or 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, and are usually Tier 1-2.