Hydrogoliath
The Hydrogoliath is descended from Goliathpseudopodias that leaned into a niche of needing little oxygen. These organisms escaped a densely-populated world of oxygen-loving, fast-moving, massive macro and near-micro predators by going deeper underground. Having developed hydrogenosomes (the lemon wedge-shaped organelle) it does not actually need oxygen at all, and can live in deeper depths. Nonetheless, it is unable to get enough energy to reproduce if the oxygen levels dip under 2%. Other than becoming better-adapted to living in the low-oxygen depths of marine silt, it's almost identical to its ancestor, both internally and externally.
Amoeba-like, it crawls between grains of sand or gravel and engulfs microbes or organic matter. As it produces hydrogen as a waste product of hydrogenosome-based metabolism, it co-occurs with Skarlix descendants (like Skarlixids) which eat hydrogen or hydrogen-based compounds and "breathe" various metals.
Hydrogoliaths can take up DNA directly from its environment and incorporate it into its own genome. While a few other uniceullar organisms of its time are capable of horizontal gene transfer, the phenomenon is more common (relatively speaking) for the Hydrogoliath. Skarlix descendants are the most common "donor", due to similar habitat and depths, and most of its genes from Skarlix descendants relate to handling hydrogen compounds.
Hydrogoliaths' "favorite food" is Protomancerxian microbes, like Crystalkillers and Gutburners, for it orients towards and engulfs Protomancerxians faster than with other microbes.