Dictyostelid
The dictyostelids are a group of cellular slime molds, or "social amoebae". They are unusual eukaryote organisms. For much of their life they live as separate cells. Then in hard times they come together to reproduce as a fruiting body. The fruiting body produces spores with protective walls. They survive and open up in better times. So these organisms are both unicellular and multicellular.