Barry who saw that widened his eyes. He didn't expect that suddenly those plant monsters could find a way to launch on of them into the air to meet him.
'I guess it's not a mindless monster like I thought.'
Since they started fighting, Barry and the others always thought that the monsters they were fighting were creatures whose minds had been taken over by whatever power was controlling them. So he didn't really think about the possibility that they would act intelligently.
So far all that is true. However, Barry's thoughts about that immediately disappeared seeing the Devourer Plant's behavior.
From the beginning the monster created an illusion to trick them by placing the huge vegetation which made its body become giant. Then, when he succeeded in destroying the monster, it changed its plan to creating another human-sized monster to fight them more effectively.
