Inspired by the answers to this question, I want to ask about the different present participles of esse over time and their fate. I am aware that esse is a defective verb that classically does not have a present participle. But there are frozen forms like absens suggesting an old form *sens for a present participle, and there is ens calqued from Greek. The word essentia "essence", another calque from Greek, also suggests a form like *essens.
In the beginning, way back in the far-flung times of Proto-Indo-European, the word for "it is" was something like *h₁ésti, and it had a fairly regular present participle, *h₁sónts. In Latin, these forms evolved into est and sōns, respectively (vowels get lengthened before -ns). The latter is where we get forms like absēns > "absent" and praesēns > "present", with vowel reduction in non-initial syllables. (This reduction evidently happened before the lengthening.)
But over time, the meaning of this word started to drift. In legal language, "the one who is" started to mean "the real one", then "the actual perpetrator", then "the guilty one", and eventually sōns became synonymous with reus. So it didn't really work as a participle any more.
People then made do without a present participle for quite a while, until they had to translate philosophical texts from Greek. The philosophical term οὐσία "essence" was formed from the Greek present participle plus the noun-forming -ία. And while the latter had a nice equivalent in Latin (-ia), the former didn't. So some translators took the infinitive esse, built a fake participle off that (*essens), and attached -ia onto the end of that, giving essentia. Essentia caught on (hence English "essence" and all its cognates), but *essens didn't.
Eventually, in Mediaeval Latin, people started to question the lack of a present participle (since Greek and Romance had one). The Ancient Greek one in particular was ὤν (also from *h₁sónts; initial
/s/ tended to vanish in Greek), which coincidentally looked just like the standard participle ending (as in λύ-ων "releasing"). So by analogy, they took the ending off the Latin present participle, and started using it as a word of its own: curr-ēns, curr-entis > ēns, entis. While it's not Classical, this is probably the most popular participle you'll find, and will be the most easily understood nowadays.
In medieval Latin there were neologisms such as ens. The link also says that the original form was sons with the classical meaning "guilty".