Most likely the offspring of "Queen Anne".
The Kid can make it follow him and stop following him by pressing E when near it. It can dig through air, as shown when the Kid tells the Anklegator to follow him and then rolls quickly across to the other side of the gap.
"It is very large and heavy, and its occupant sounds to be alive.
Anklegators were thought to be extinct, because their eggs took decades to hatch and were easy prey.
The Calamity must have unearthed this one."
To Rucks: "Ever seen anything like this before?"
- "They say Queen Anne was just a folktale. Now there's livin' proof she ain't."
To Zia: "Do you think an Anklegator could be tamed?"
- "That gator's a crazy gal, but so's the gal that promised t'look after her.""