Originally started after the bancruptcy of Flagship Studios and the resulting shutdown of the official servers in January 2008 the goal of the Hellgate Revival project was to bring back the multiplayer aspect of Hellgate: London.
Pets are now available including Cokomoko & Mantrawraith.
Do they grant any bonuses or battle aid?
No, they were purely for cosmetic purposes. They were given to those who pre-ordered the game or found one of the registration codes in issues of PC Gamer.