Neptune was a colossal Kaiju who attempted to destroy New York headquarters. Unlike other Kaiju, Neptune displayed a reasonable amount of intellect. He avoided Jaegers and headed straight for the headquarters. Taking this into account, one could assume Neptune was created specifically by the Precursors as a "tamer" of the Kaiju. Not enough intelligence was shown to test this theory though.
Neptune was truly gigantic, even by Kaiju standards. Its height could be estimated to be around 1200 ft. Its appearance was more humanoid than most other Kaiju and was, as a survivor of his first attack described, "Like a centaur", with its body ending in 4 crab like legs. Many tentacles emerged from his back, making it similar to a squid. Its entire body was covered in fish-like scales. Its face was hairless and covered in tentacles. Its eyes were reptillian and its teeth razor sharp.
The appearance of Neptune is terrifying to those studying Kaiju. Its high radiation level and gas production could spell an end to "Kaiju Medicine", treatment to those affected by Kaiju Blue. Its Kaiju Skinmites were noticeably more scorpion-like and large. This could either suggest that Neptune is vital enough to support larger Skinmites or that larger Kaiju exist to support these "Behemoth" Skinmites. Either way it is not good. The Behemoth Skinmites were around 6 feet long and were not aggressive. The Kaiju cultists call Neptune the Queen of the Kaiju Hive. According to their scripture this means that it is time for the Kaiju, or Old Ones, to take back the Earth. This does not mean prophetical truth but instead possible riots.