We had a small discussion
in June on the whereabouts of the Thundering Serpent Hatchling
, but unfortunately never came to a firm conclusion.
It should be
on the BMAH loot table -- probably showing up very, very, very, very rarely. It's difficult to tell whether or not that's actually the case though.
Since the pet is not cageable and BoP, there's no other way to collect this pet other than buying it from the BMAH. =\