They only started doing the tribe symbols with Pearl Islands where you can see the captain's faces on their flags. Then in all-stars it was solidified and ever since that's been a standard.

Vanuatu - Bat and Turtle
Palau - Shark and Stingray
Guatemala - Jaguar and Alligator
Exile Island - Mermaid, Fortune Teller, Snake, Alligator
Cook Islands - Dolphin, Lizard, Turtle, Stingray
Fiji - Axe and Spear
China - Dragon and Tiger
Micronesia - Octopus and Snake

Also it's interesting to see where they got the names of tribes:

Borneo - Island Names (beaches)
Outback - Animals (kangaroo and crocodile)
Africa - African Tribes
Marquesas - Water and Sun
Thailand - Phrases
Amazon - Animals (bird and fish)
All-stars - Island Names

Vanuatu - Volcano Names
Palau - Island Names
Guatemala - Lake Names
Exile Island - Island Names
Cook Islands - Island Names
Fiji - ?
China - Animals (fighting tiger, flying dragon)
Micronesia - Island Names