Do you mean the “Elephant Roundup” as displayed on the Mauryan Civilizations/History page in game? Unfortunately the civilization overviews contain a lot of misinformation. There is a patch for that ( https://code.wildfiregames.com/D2720 ), but it needs to be reviewed before it can be included in the development version.
Information displayed in the Structure Tree is correct.
Organic units can't be captured. You can only lure an elephant by attacking it with a ranged unit, which makes the elephant to follow it to wherever you want. Then you can kill the elephant with other units, provided that you keep the luring unit in a safe distance.