While parts of the original Harry are still in Mastiff's appearance beneath his mien the bestial traits of his seeming are far more pronounced than the majority of his original human self. His skin is closer to fur than human flesh and there's even traces of feathers at his neck though not especially many in comparison to the rest of his body. His frame is slightly hunched as though eager to be on four legs rather than two, his teeth and nails are closer to fangs and claws than what you would expect for any biped in the natural world and atop his brow rests antlers, larger than those found on goats or sheep but being closer to those of a deer than an elk.

Step 6: See You On The Other Side

The original Harry remembers who he was but these days he tends to define himself as what he is now. Offered as an ideal bit of prey to The Noble in Red he was taken in as part of The Noble's Great Hunt, a constantly changing menagerie of beings hunting or being hunted. Prey had a chance to become predator and vice versa as The Noble alternated between leader of the hunt, casual watcher or celebrant dancing through the bloodshed. While at first Harry struggled constantly to escape the life of the hunted and even after becoming a hunter initially refused to hunt those he had left behind. A return to the life as one of the targets of the hunt eventually crushed that and next time he got the chance to be a predator he went all in on being the best he could. In fact he was so good a predator that when the prey he was hunting got away from The Noble they did not get away from him. He only stopped pursuing when he realised the chase had gotten them both away.

RP Guide

Mastiff's still got traces of Harry as he was, with a hefty sense of ego invested in areas he excels. His time as a captive though those talents were the difference between being a predator or prey so he's much less of a good loser. He tends to see competition where there is none and while he's capable of working with others if he can see an opportunity for him to pull ahead he'll take it.
He's aware he does this though and knows it comes from his durance so when he's not acting impulsively he tends to watch his own actions with a critical eye. The hunts he was part of are not who he wants to be going forwards, but instinctively he still falls back to that sort of behaviour.
One of his more vulnerable areas from before being taken is despite being decent enough in his sporting performances that there was a chance he could pursue it as a career he also knew when he watched the performances of others that he probably did not quite have what it took. He was good but even working as hard as he could others were just that bit ahead of him. While he never got chance to  find out for sure the fact that his escape from captivity was pursuing someone else's back is not something that he has failed to realise.

Icons

Collar - a more impressive name than it actually deserves this is a plain length of chain, sturdy despite looking like it had been used for so many captives over the years that the blood from where the restraint chafed them has soaked into the material itself. A fairly common accessory for any of the Noble In Red's captives and carries with it some of the decent memories of his time there, snatched moments between hunts or when alongside others where those that still had memories of their time beyond exchanged stories, memories or just random conversation in an attempt to carve some semblance of normal life amongst what was happening. Without these not only are his memories of those fellows lacking Mastiff's ego is more unchecked, feeling he managed to pull through largely on his own. In some ways that helps him cope since he feels stronger for having managed to keep part of himself where so many did not but it also means he's more contemptuous of the weakeness of others, seeing it as signs of inevitable failure.

Hunting Horn - crafted from the horn of a large animal with minimal elegance or decoration to it. This was formed when he was first taken and represents greater ambitions for him. The time was that he had plans for leaving his hometown and going onto bigger things than were possible if he remained there. Without it he's a tendency towards tunnel vision, seeing only simple goals and simple solutions rather than looking past them at potential greater overall objectives.