Circus staff are said to be devastated by the loss of the popular pair on Sunday night, while staff were distracted erecting tents.
Staff are certain the animals could not have could not have come loose themselves, but are uncertain whether troublemakers simply untethered the animals or stole them.
Belle performs her own set of circus tricks, while Dusty performs in a group act.
The animals are expensive to train, acquiring skills of little use to anyone else.
The circus is keen to get them back.
Belle is a miniature palomino mare. She stands 111cm high and was wearing a white rug and a blue halter. Dusty is shorter, around 70cm high.