Is there a such thing as miniature old English sheepdog?

Yes, miniature old English Sheepdog is a real thing.

It was developed in the United States in the 1970s by selectively breeding small Old English Sheepdogs. Miniature Old English Sheepdogs are typically between 12 and 18 inches tall and weigh between 20 and 30 pounds.

They have the same gentle and friendly personality as their larger cousins, but they are more adaptable to apartment living and other smaller spaces. Miniature Old English Sheepdogs make excellent companion dogs and are also well-suited for activities such as agility and obedience.