Most grammar textbooks will tell you that the Latin ‘to be’ has only a future active participle. On a practical level, that’s true. However, there is evidence within the Latin language of a lost present active participle. This would have been sōns, sontis. (cf. Greek ὤν).
However, this form is all but lost. We may conjecture that it existed at one time because it is stored in certain adjectives (īnsōns, innocent; absēns, absent, praesēns, present). It also appears in late Latin philosophical terminology (ēns, being; entia, the things which are). However, these were likely designed by intellectuals to reflect the present participle as it would appear, were it in use. Honestly, the same might be true of insōns, etc, but with words that old, we can’t trace their origins properly.
The Essential AG: 170b
Latin for Addicts 