A heavily slowed down dog bark could be a start, but you need some power with something like a cow/bull. Some sort of hiss/wind as well, because this thing must project quite a lot of air! An elephant could help and why not a siren (WW2 kind of bombing alert).

This is just brain storming, I might come back with more! :)

A heavily slowed down dog bark could be a start, but you need some power with something like a cow/bull. Some sort of hiss/wind as well, because this thing must project quite a lot of air! An elephant could help and why not a siren (WW2 kind of bombing alert).

This is just brain storming, I might come back with more! :)

EDIT: revving up an engine could add character to the roar?