Ex-UFC star Frank Shamrock is reportedly being investigated by cops after he tied a dog to a truck and abandoned it for 5 DAYS at a Dallas airport earlier this month.
Shamrock had to move his ailing mother out of her Texas home -- and in the process, was forced to take control of her two dogs.
Frank tells WFAA in Dallas he was able to find a shelter for one of the dogs ... but when he couldn't find a home for Zelda, the other dog -- he tied her to a truck and left her at Love Field airport.