The animal rescue group said there was a cardboard box nearby the iguana with some vegetables in it, indicating the reptile may have been abandoned at the side of the road and managed to escape the box before being spotted by the dog walker.
The RSPCA said the iguana was in good physical condition but needed to be warmed up due to the cold.
"It was really lucky that the iguana was spotted as it was quite a remote location and would not have survived long in the cold," RSPCA inspector Lucy Green said.
Green said the iguana may have been abandoned by a pet owner who decided they did not know how to properly care for the reptile.
"Very sadly, many people buy exotic pets without thinking through the responsibility first and with little or no idea of how difficult they are to care for, which as a result can lead to them being abandoned in a callous way like this. Ultimately it is the animal that suffers after the novelty wears off," she said.
Green said she is trying to identify the lizard's former owner.
"I am concerned that whoever had the iguana may have other pets like fish, and am worried about their ability to care for them," she said.
The RSPCA said the iguana being cared for by a reptile specialist.