Q:

Ancient DNA of ancestors or existing species?

Ancient DNA appears to be at most "several hundred thousand years" old. 
Do we have an aDNA sample which we can reasonably confidently attribute to a species which has at least two modern descendants?
PS. This question is much less interesting than I originally thought: there are modern species which are proven ancestors of other multiple modern species (lake Victoria Cichlids).

A:

Assuming you mean by "modern descendants" "two different species that are also different from the ancient DNA", then to the best of my knowledge, no; the large vertebrates (bear, horse, humans, mammoths) that have been focused on for the most ancient DNA samples have common ancestors that are outside the range of ancient DNA isolation. I'm not very familiar with plant ancient DNA; maize DNA from a few thousand years ago is the oldest I'm aware of, but I might easily have missed some. 
However, it's important to point out that the way this question is phrased makes it merely trivia, with no scientific significance.  
Assuming (from the poster's other questions) that he believes this is somehow relevant to questions about speciation, I would point to modern genomes from ongoing speciation.  For example, Heliconius butterflies are speciating, and there's some work looking at the genomic components of that speciation:

Highly conserved gene order and numerous novel repetitive elements in genomic regions linked to wing pattern variation in Heliconius butterflies.
Genomic islands of divergence in hybridizing Heliconius butterflies identified by large-scale targeted sequencing.
Divergence with gene flow across a speciation continuum of Heliconius butterflies

Stickleback fish also show a number of examples of incipient evolution, and again genome studies have been done:

(Genomic landscape of early ecological speciation initiated by selection on nuptial colour.](http://onlinelibrary.wiley.com/doi/10.1111/mec.13774/abstract;jsessionid=DB86FB85259118805B95C1C52138D3EA.f03t03)
Genomics of Rapid Incipient Speciation in Sympatric Threespine Stickleback.

Since, as I say, the question as phrased is simply asking for scientifically unimportant trivia, I'll leave it that as far as guessing as to what the actual point is.

