harry potter - Why did Grindelwald have Antonio in his cell?

We know that Grindelwald had a baby Chupacabra chained to his chair in his cell.

A baby Chupacabra—part lizard, part homunculus, a blood-sucking creature of the Americas—is chained to GRINDELWALD’S chair.

_{Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay, Scene 2}

We also know it was called Antonio and was, most likely, killed during Grindelwald's escape... by Grindelwald. It also appears to have been part of the escape plan.

GRINDELWALD rips the little Chupacabra off SPIELMAN. It rubs its bloody face affectionately against his hand.

GRINDELWALD I know. Okay. I know, Antonio.

He looks at it with distaste.

GRINDELWALD So needy.

He then flings it through the door.

_{Fantastic Beasts: The Crimes of Grindelwald - The Original Screenplay, Scene 15}

But, why was Antonio in the cell with Grindelwald in the first place?

Answer

It seems likely that Antonio was part of the security measures around Grindelwald. When he’s chained to the chair, Grindelwald is also bound to the chair and surrounded by protective spells.

SCENE 2
INT. MACUSA BASEMENT, BARE, BLACK-WALLED ROOM—NIGHT

The long-haired, bearded GRINDELWALD sits motionless, magically fixed to a chair. The air shimmers, charged with spells. ABERNATHY peers in at GRINDELWALD from the corridor. A baby Chupacabra—part lizard, part homunculus, a blood-sucking creature of the Americas—is chained to GRINDELWALD’S chair.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

This all seems to be protective measures to keep Grindelwald from escaping. They were about to transport him to Europe, and had thrown everything at him to keep him secured.

As they reach the door, ABERNATHY turns and acknowledges them.

ABERNATHY
President Picquery, Mr. Spielman, sir. Prisoner is secured and ready to travel.

SPIELMAN and PICQUERY peer into the cell at GRINDELWALD.

SPIELMAN
You’ve thrown everything at him, I see.

PICQUERY
It was necessary. He’s extremely powerful. We’ve had to change his guard three times—he’s very . . . persuasive. So we removed his tongue.
_{^{- Fantastic Beasts: The Crimes of Grindelwald (The Original Screenplay)}}

Because of this, it seems likely that Antonio was a security measure as well. Picquery said that they’ve had to change Grindelwald’s guard three times because he’s persuasive. Antonio may have been there either as a guard he shouldn’t be able to persuade, or to prevent anyone he did manage to persuade from getting too close to Grindelwald to set him free. However, if Abernathy was the one who decided or suggested putting Antonio to guard Grindelwald, it may have been his plan all along that Antonio would actually end up helping in the escape, since Abernathy was actually working for Grindelwald and helped him to escape from custody. It did seem to be considered a reasonable security measure by the others, though, since no one really questions it.