"A plant is a plant, and an animal is an animal. It is impossible to be both at the same time, even if plants behave like animals and animals remain as plants.

Of the two scenarios, the second one undoubtedly seems the less unlikely. In this sense, Ents would be animals whose external appearance is that of a plant.

The Ents would ultimately be like animal-plant chimeras populating Middle Earth, sometimes as giant photosynthetic phasms, sometimes as terrestrial branching corals."

 (op.cit, La possibilité d'un ent).

Plant? Animal? It is difficult to decide for Ents... and this is also sometimes the case for certain fossil or living organisms.

Corals, which can sometimes look like plants, are animals that are similar to jellyfish, and fungi are actually more similar to animals.

Sea lilies, which look like flowers as their name suggests, are closely related to sea urchins. Bacteria are neither animals nor plants but separated themselves from the living world before the appearance of common animals and plants. As for the viruses, some might say that they are not even alive!