The animal welfare charity needed assistance to create a safe route for the horse, which was said to be in a bad way, a fire service spokesperson said.
Littlehampton fire crew used an electric saw and small tools to free the horse from the field, just north of Rustington on the A259.
It is not known exactly how long the horse had been left in the field, but the animal is now being cared for by the RSPCA.
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