An Antarean Bogg is an alien species of unknown origin.
Based on Jake's description in The Invasion when witnessing Visser Three morphed into one, Antarean Bogg are huge and monstrous alien creatures with two legs that are as big and as round as at redwood tree, a huge bloated head with a wide, monstrous mouth filled with teeth as long as a human arm, and thick tentacles for arms. In TV Series however, their appearance changed into a humanoid reptilian beast that greatly resembles Hork-Bajir, except that they have shorter neck and more lizard-like head.
Visser Three possesses Antarean Bogg morph in Book #1: The Invasion, which he infamously used to devour and kill Elfangor. Before he did so, he had some of his Controllers hold the Andalite so he cannot escape his doom.
Visser Three uses Antarean Bogg morph to kill Elfangor in the same manner in the book, though he doesn't need to had some Controller's help to restrain the Andalite as the Andalite is already dying. He later used it again when one of his Controller servants insults him.