The Portuguese Man o’ War is a colony of four different organisms which form a composite jellyfish; None of the individuals which can exist in a free-living form. Lichen is a colony of two: a photosynthetic partner providing sugars, and a fungus providing other nutrients as well as preventing the dehydration of the photosynthetic partner.
Hatena is another such example, but with its own twist. Like many unicellular organisms Hatena vacuums up smaller organisms and nutrients as it swims through an aqueous medium. But when it ingests a certain alga, Hatena and the alga go through a fascinating metamorphosis. The alga within is not digested but rather grows while losing some of its components, and Hatena takes on alga-like qualities, making it less like a predator and more like an autotroph. For instance, its mouth is replaced with an alga-like light sensor, biasing Hatena's motions toward light which is needed for the alga within to perform its photosynthesis.
Interestingly, when Hatena reproduces it creates a Cain and Abel scenario, with one of the progeny turning back to the predator design and the other inheriting the alga and continuing as an autotroph.
Hatena is reminiscent of Euglena gracilis which can perform photosynthesis if sunlight is present, but also is mobile and consumes nutrients if they are available. It could be classified as a plant or an animal. But Hatena's metamorphosis is triggered not by the environment but by what it eats.