Advance a bombadier beetle and drop the acid spray, maybe add a conditional Hide bonus? The wikipedia article suggests giving them a really high speed (I guess the original stats do, too!).
Yes, real-world tiger beetles can be very fast. I'd suggest speed 40 or 50 ft. plus Run as a bonus feat.
If they're fast, we could give them a higher Dex to help explain why they have an impressive AC for a Medium 3HD beetle.
I'm thinking something like the following, based on the Giant Stink Beetle we just did:
Tiger Beetle, Giant
Medium Vermin
Hit Dice: 3d8+3 (16 hp)
Initiative: +2
Speed: 40-50 ft.
?
Armor Class: 17 (+5 natural, +2 Dex), touch 12, flat-footed 15
Base Attack/Grapple: +2/+4
Attack: Bite +4 melee (2d6+3)
Full Attack: Bite +4 melee (2d6+3)
Space/Reach: 5 ft./5 ft.
Special Attacks: -
Special Qualities: Darkvision 60 ft., vermin traits
Saves: Fort +4, Ref +5, Will +1
Abilities: Str 14, Dex
14, Con 13, Int —, Wis 10, Cha 0
Feats: Run (B)
Many Tiger Beetles also have well-developed eyes, so maybe a racial bonus on Spot checks?