Shloosh
Shloosh was a Mycon who was incinerated 14,000 earth years ago. During The Captain's mostly unsuccessful attempts to get any sense out of the Mycon, Shloosh appeared to briefly posess a Mycon Podship captain that he was talking with, to state "I live now for but a moment; then, I am gone.". The purpose or reasoning for this temporary interruption on his part is, like so much else with the Mycon, unknowable.
Interestingly, given the immense amount of time ago when Shloosh claims to have been alive in a corporeal sense, he may have been alive during the time of either the Sentient Milieu or at least its Dnyarri controlled state.