Here's why:

* Eukaryotic: Euglena have a true nucleus, unlike bacteria which are prokaryotic.

* Single-celled: They are microscopic organisms consisting of a single cell.

* Mixotrophic: They can obtain energy through photosynthesis (like plants) but also through consuming other organisms (like animals). This unique combination makes it difficult to classify them definitively as either plant or animal.

Therefore, they are placed in the kingdom Protista, which includes a diverse range of mostly single-celled eukaryotic organisms.