This is Simon’s Imperial Guard Hellhound.
He has made a few changes which make it look more like the Churchill Crocodile variant from the Second World War. Rather than have the external fuel tanks on the back of the Chimera, he has added an external fuel trailer. This is from the Ork Skorcha model with an additional fuel drum added. In combat this would have the advantage that if the main flame weapon was taken out (or ran out) they could drop the trailer for greater speed and better manoeuvrability.
In our games we do use the Apocalypse flame template and this earlier posting explains why.
More pictures of the Hellhound.