Eukaryote without mitochondria

pubmed.ncbi.nlm.nih.gov/27185558/ A Eukaryote without a Mitochondrial Organelle by Karnkowska et. al (2016)

As of 2020, there are no known eukaryotes which have never had mitochondria.

Known eukaryotes without mitochondria, which are very rare, have lost mitochondria they previously had.

Having mitochondria appears to be a requisite for being an eukaryote. This is one of the central thesis of Power, Sex, Suicide by Nick Lane (2006).