Officers were informed that a number of pellets had been thrown over the fence into the hydraulic fracturing site on Monday morning.
A guard dog at the site was taken to a vet for treatment after eating some of the pellets and then vomiting and collapsing.
Several pellets were seized from the site which North Yorkshire Police have now revealed are suspected to be aniseed balls.
A spokesman for the force said: “The dog received treatment at a vet and is recovering well.
“The pellets retrieved from the ground at Kirby Misperton are suspected to be aniseed balls. Enquiries are still ongoing.”
A caravan parked on Habton Road, Kirby Misperton, was searched as part of the investigation and two men were arrested on Monday.
A 42-year-old man was arrested on suspicion of administering poison to an animal and causing criminal damage to an animal, and a 61-year-old man was arrested on suspicion of causing criminal damage to an animal.
Both men have been released on bail, with a condition to stay away from an area around the site gates, as police enquiries continue.