General synonyms:
* Wild
* Feral
* Unruly
* Uncontrolled
* Unrestrained
* Savage
* Primitive
* Untamed
* Uncultivated
* Unbridled
* Unrestrained
* Free-spirited
Synonyms for specific contexts:
* For animals: Wild, feral, untamed, undomesticated
* For nature: Untamed, wild, rugged, untamed, raw
* For people: Unruly, rebellious, uncontrolled, unrestrained, free-spirited
The best synonym will depend on the specific situation and the meaning you want to convey.