Lucius found himself mildly impressed by the speed at which Desmond took down the Ent all on his own. Granted, he did not know the monster was called that until everyone told him after the fact. Since he had spent such a long time in the deserts of Alorek and jungles of Eroa, he was most familiar with the flora and fauna of those areas.
Back when he was an adventurer, he learned about monsters as they faced them, so he never really spent a lot of time learning about them. Rena was sure to chastise him for his behavior and demanded he devote more time to studying things like this especially since he was a student now.