Frank Shamrock says he didn't mean to hurt the dog he tied to a truck and left at an airport ... telling TMZ Sports, "I know what I did was wrong, but there was no way out of it."
As we previously reported, the UFC legend is under investigation after cops say he abandoned a dog at Dallas Love Field Airport ... leaving it tethered to the bed of a pickup truck for 5 days.
Now, Shamrock is explaining to us why he did it ... saying, "I had no other solution to solve that problem."
The former fighter tells us his mother is ailing, and when he traveled to Texas earlier this month to take care of her ... he realized she was in no condition to own dogs anymore.
Shamrock says he tried his best to find new homes for the animals ... but just could not find a place for Zelda.
"I called everybody in Dallas, I tried everything humanly possible to get care for the dog, to let people know that we could not take the dog."
Shamrock says his search for a new home for Zelda lasted up until just minutes before he had to board a plane back home ... so he decided the best thing to do was to tie her up with food and water and call security to come get her.
The problem? Security never found her, and she was stuck in the pickup bed for 5 days before police officers rescued her.
Now, Frank tells us he's heartbroken over the whole ordeal ... mostly because he can't believe people at the airport let her stay in the truck for that long without getting her.
"The dog stayed there for five days. I can't believe that," Shamrock said through tears ... "That wasn't what I was intending. I thought they would come get it."
Frank is now under investigation by the Dallas Police Department, who tell us they are expecting to file a charge of Cruelty to Non-Livestock Animals -- a Class A Misdemeanor -- against the former fighter.
Shamrock says he'll accept any charges, telling us, "I take responsibility for what has happened."
As for Zelda, a spokesperson for the SPCA of Texas tells us she's doing just fine since the incident -- and has received "a ton of TLC" and care while she's been in the organization's custody.
The spokesperson also tells us they expect to put Zelda up for adoption within the next few days.