The little Welsh pony, named Puddles by rescuers, is thought to have been trapped in the ditch for days before she was spotted by a member of the public.
She was winched to safety by Essex Fire and Rescue Service and RSPCA officers.
Puddles, who is thought to be 15 years old and is around 3ft 8in tall, is being cared for by a vet until her owner can be traced.
She is not microchipped.
RSPCA inspector Caroline Richardson said: “It’s amazing that she was spotted as she was absolutely covered head to toe in mud and was neck-high in muddy water.
“You could just see her ears, which were the only part of her that were still grey.
“It’s total chance that she was found and she was very lucky to have been spotted.
“She was freezing cold, shivering and clearly exhausted.
“We believe she may have been stuck in the ditch for days and, with the weather having turned so cold and wet, I don’t think she’d have survived much longer.
“She’s a very lucky little pony.”
She said that although Puddles was “a little stiff and terrified”, she was unharmed.
Ms Richardson said that if the pony’s owners are not found she will be rehomed.
Anyone who recognises Puddles or believes her to belong to them is asked to call the RSPCA’s appeal line on 0300 123 8018.