So I'm a little confused by the Giant Weasel stats, as I'm looking to use it as a companion for my halfling sylvan sorcerer.
RoW Bestiary:
Large, Str 19, Dex 17, Con 18, Int 2, Wis 12, Cha 11
40 ft, 20 ft climb, bite 1d8, attach, bleed 1d4, scent
RoW Animal Companion (starting):
Medium, Str 13, Dex 19, Con 13, Int 2, Wis 12, Cha 11
40 ft, bite 1d6, 2 claws 1d4, scent
RoW Animal Companion (7th level):
Large, Str 21, Dex 17, Con 17, Int 2, Wis 12, Cha 11
40 ft, bite 1d8, 2 claws 1d6, attach, scent
Bestiary 4 Entry:
Medium, Str 14, Dex 19, Con 10, Int 2, Wis 13, Cha 11
40 ft, bite 1d6, blood drain (1d2 con), grab, compression, scent
B4 Animal Companion (starting):
Small, Str 10, Dex 19, Con 13, Int 2, Wis 12, Cha 10
30 ft, 10 ft climb, bite 1d4, blood drain (1 con), grab, scent
B4 Animal Companion (4th level):
Medium, Str 14, Dex 17, Con 15, Int 2, Wis 12, Cha 10
30 ft, 10 ft climb, bite 1d6, blood drain (1 con), grab, scent
The inconsistency here is.. frustrating. Why are the AC weasels stat'd completely different than naturally occurring ones? I get some variation. Blood Drain wasn't a thing with the original in RoW, so there's bleed instead. The size differences, the mental stat differences, the compression differences, the claws - all seems like this creature has got some serious identity issues.
By comparison, a wolf starts off matching the bestiary entry, and then grows from there (gaining appropriate abilities - str and con up, dex down).
Wolf
Medium, Str 13, Dex 15, Con 15, Int 2, Wis 12, Cha 6
50 ft, bite 1d6, trip, scent
AC (starting):
Medium, Str 13, Dex 15, Con 15, Int 2, Wis 12, Cha 6
50 ft, bite 1d6, trip, scent
AC (7th level):
Large, Str 21, Dex 13, Con 19, Int 2, Wis 12, Cha 6
50 ft, bite 1d8, trip, scent
Most animal companions have different stats from their bestiary counterparts, because giving the animals from the bestiary as companions would be too powerful at low levels.
Take a look at the big cat animal companion vs a tiger in the bestiary.
There's no rules problem here.
Wolves are one of the few animal companions that closely match because they start out as CR 1 creatures and are relatively weak.
looks like they changed their mind on the stats to give it between RoW and B4 - i think usually the bestiary is the most recent version of the creature so that's probably the one you should use
Most animal companions have different stats from their bestiary counterparts, because giving the animals from the bestiary as companions would be too powerful at low levels.
Take a look at the big cat animal companion vs a tiger in the bestiary.
There's no rules problem here.
Wolves are one of the few animal companions that closely match because they start out as CR 1 creatures and are relatively weak.
I get that they do that at times - these differences are just weird, and don't all seem done for balance. It's just odd, and seems more like arbitrary stuff where the stats were simply made by different people. Even if the RoW stats are thrown out - the major changes between the bestiary monster and the mature Companion:
Companion loses 10 ft of speed.Companion gains a 10 ft climb speed, but loses compression.
Companion loses 2 DEX, but gains 5 CON
Companion loses 1 WIS and 1 CHA (with no mechanical effect)
Companion loses half its blood drain damage (1d2 to 1)
None of these seem extensive or balance based, just strange.