To start, A LV1 Wizard gets a familiar, which levels up to equal each odd Wizard level (LV3 and Wizard LV3, up to LV19 at Wizard LV19). Also, familiars are animals except for spiders and beetles which are virmin. This affects rangers favored enemies and perhaps succeptability to hold animal vs hold monster spells (althought I didn't test that).
CASTER BONUS (while familiar is summoned)
Bat +3 Listen
Cat +3 Hide, +3 Move Silently
Rat +2 Fortitude Save
Rabit +1 All Saves
Weasel +2 Reflex Save
Spider +3 Bluff
Pig +3 Diplomacy
Beetle +1 HP per character level
ABILITY SCORES
All familiars start at 6 Intelegence which is the only ability which advances by one at each level up to 15 at LV19.
LV 1 3 5 7
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IN 6 7 8 9 10 11 12 13 14 15
The rest of the abilities are fixed
Familiar ST DX CO WI CH
Bat
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Cat
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Rat
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Rabit
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Weasel
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Spider
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Pig
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Beetle
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The animals have the highest armor class and lowest hit points. The beetle has the lowest armor class and highest hit points. Spiders are in between.
ARMOR CLASS (including DX bonus)
LEVEL
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Animal 15 16 17 18 19 20 21 22 23 24
Spider 13 14 15 16 17 18 19 20 21 22
Beetle 11 12 13 14 15 16 17 18 19 20
HIT POINTS (including CO bonus)
LEVEL
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Animal 4 12 20 28 36 44 52
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Spider 5 15 25 35 45 55 65
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Beetle 7 21 35 49 63 77 91 105 119 133
SAVING THROWS (excluding ability bonus)
Animals have Fortitude and Reflex and high saves. Virmin only have Fortitude as a high save
LVL 1 3 5 7 9 11 13 15 17 19
Hig 2 3 4 5 6
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Low 0 1 1 2 3
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The following statistics are the same for all familiars (excuding ability bonuses): SpR Spell Resistance, BAB Base Attack Bonus, Hid Hide, MoS Move Silently.
LVL 1 3 5
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SpR 0 0 0
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BAB 0 2 3
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Hid 4 6 8 10 12 14 16 18 20 22 Note at LV1 the Bat has 0 Hid
MoS 4 6 8 10 12 14 16 18 20 22 Note at LV2 the Bat has 6 MoS
SPOT LISTEN (excluding ability bonuses)
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Bat
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Weasel 0 2
Spider 0 4
Pig
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Others 0 0
FEATS
All familiars have Weapon Proficiency: Creature, and Improved Evasion
Cat, Rat, and Rabit aslo have Weapon Finesse
Weasel, Spider, and Beetle also have Low Light Vision
Bat and Pig have no third feat
DAMAGE (including ST bonus)
Animals 1
Spider 1-2
Beetle 2-3
Critical 20 x2