Is there is any external source about Demas, Paul's fellow worker? Demas appears at Colossians 4:14 and Philemon 24 as a fellow worker of Paul's ministry, and maybe as Luke's partner like Mark was Aristharcus partner. 

But in 2 Timothy 4:10 he appears having abandoned Paul. There is any tradition source about this story? Any ancient reliable source about Demas?
Demas is mentioned in the Acts of Paul and Thecla, an apocryphal work attested by Tertullian around AD 190.  

In this work, Demas is portrayed as a troublesome "follower" of Paul.  Paul is portrayed as teaching the value of chastity, and Demas opposes him:


  [Paul] deprives young men of wives, and maidens of husbands, saying, There is for you a resurrection in no other way, unless you remain chaste, and pollute not the flesh, but keep it chaste.


Demas encourages a young man, whose fiancee has been influenced by Paul's teaching, to oppose Paul before the governor, who then scourges Paul and throws him out of the city.

Other than this account, there does not appear to be much tradition related to Demas.  A man by that name is also mentioned in On the Seventy Disciples, as one who "became a priest of idols."  But he has no entry in the Catholic Encyclopedia, nor the Encyclopedia of Ancient Christianity, suggesting that little more is known about him.