General synonyms:
* Speckled
* Spotted
* Freckled
* Dappled
* Marbled
* Streaked
* Patchy
* Variegated
* Stained
* Discolored
* Irregular
* Uneven
More specific synonyms:
* Brindled: (used for animals with stripes or patches of different colors)
* Calico: (used for cats with patches of different colors)
* Piebald: (used for horses with patches of white and another color)
* Roan: (used for horses with a mixture of white hairs with another color)
The best synonym for "mottled" will depend on the specific context and the type of pattern you are describing.