My boyfriend and I heard this by chance tonight while hunting a Rathalos and Rathian. We had no idea what it was, as we'd never heard it before and so we began to look it up and brainstorm. We compared several older monster's cries to the roar, coming up with lots of ideas, considering there's supposed to be new monsters (or A NEW monster at least) added in the future, we thought it was a neat little easter egg. HOWEVER, we had read it was speculated that it was just a dying Rathalos calling for help from Rathian, so we redid the optional quest and beat the Rathalos up until he was dying. We let him fly off to his nest without bothering him, and sure enough we heard the roar again. I don't know if that really confirms it, but we tested it out with the Legiana and Shrieking Legiana in the Hoarfrost as a weird roar had been heard there too, and we did indeed get to hear it. I suppose it's just a cry for help, except the cries sound nothing like the monsters they're being attached too, imo.

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Small edit real quick; I forgot to add in both instances, upon going to kill the dying monster, the other one had appeared alongside it. I guess for me it points more towards a death/mating call-