Ens ut nomen and ens ut participium are found in medieval works, used in both metaphysical and descriptive senses: "existence or reality thought of as things" as opposed to "existence or reality thought of as action/process".
But, as an attempt at classical Latin, your version looks completely wrong to me.
In classical Latin the verb sum famously has no present participle although, had it existed, it would very probably have been ens, entis, which is echoed in, for instance, praesens. Lewis & Short quote ens as "a thing; formed, like essentia, after the Gr. ousia, by Flavius (or Fabianus), [and] acc. to Quint. 8, 3, 33 its first recorded appearance was in an attribution by Priscian to Caesar." Smith and (earlier) Riddle, their predecessors, follow Forcellini with something similar. All refer, I believe, to the first recorded usage from classical literature of ens in the sense ens ut participium. I can't give chapter and verse for this without a deal of research, but the source may be Caesar's supposed regretting the lack of a present participle for sum in his lost treatise on grammatical analogy. It seems to me quite wrong to rely on this single reference; perhaps the better word to use is animal or even animans, as Ovid (Met. XV, 90) has in alterius animantem animantis vivere leto (there are similar usages in other authors).
The adjective that you were thinking of is not ultimatus, but ultimus (the superlative of a supposed obselete word ultus). However, postremus, the superlative of posterus is nearer to what you want.
I suggest that a more apt translation would be animans postremus, for which you can vary the gender according to your actual context.
