Wow, well travelled horse, from Nth Qld to Vic to NT! I bet all the trucking, as it tends to go with abrupt, if not rough handling, esp if he was shipped on a doggers load, has had a bit to do with his attitudes.
The 'horsanality' concepts are not actually Parelli inventions, but behavioural psych terms that have just been applied to horses & packaged as a 'Parelli Wonder'. Eg. people/horses/dogs/whatever can be introverted or extroverted personalities. Meaning that they're quiet, 'shy', internalise things, tend to 'shut down' when under pressure, or they're 'out there', vocal, energetic, liable to 'come out swinging' when things are too much. And 'right brained' is the term for being more emotional, reactive, sensitive, while 'left brained' is more calm, rational, thoughtful. This comes from neurological studies which show the different hemispheres of the brain control different aspects of 'personality', emotions, thinking.
**All aminals can be introverted or extroverted in different situations, all are 'left brained' & 'right brained' in different situations. That is one thing a lot of Parelli followers tend to forget/misunderstand. You can't put a horse(or person, or dog...) in a 'behavioural box' and lable it as something definite. IMO that will always be incorrect. But to understand different aspects and the *motivations behind behaviours*, be it in behavioural terms or otherwise, is helpful IMO.
Anyway... agree basically with what everyone else has said. I'd be working in a low key, non confrontational manner, being aware of his smallest bodylanguage signals, to consider what he may be ready for. THAT is a huge difference between 'introverted' & extroverted horses. It is generally plain obvious with extroverts, what they thing of any given thing, whereas you might mistake 'quiet' for 'accepting' in an introverted horse, and miss the often tiny(holding breath for eg) signs that tell you he's likely to explode(or shut down) if you 'push' any harder.
Working at liberty(real liberty, in a large yard/arena, not little pen) can be great, because if they've got the freedom to 'escape' but choose not to, you know you're doing it right. If they want to escape from you, you know you're pushing too hard(I wouldn't be working on actually driving him away until you've got his trust & the basics established).
Speaking from personal experience... I had successfully trained many horses, thought I was pretty good at 'listening' to them, but in hindsight, not sure that I had much at all to do with fearful introverted types. Until one of mine was sent to someone else for training(long story...). He'd always been outgoing & easygoing & I'd already started him under saddle, he was supposed to be going for some 'icing' training that I didn't have the time for. He came back a real mess.
Uncatchable, headshy, nervous of anyone & terrified of men, a bundle of quivering nerves, even trying to do 'basic' stuff with him he'd been great at. So I basically had to start him again. Afraid to say, I did miss many signs, and thought he was ready for more at any given point... he stopped reacting, became quiet, compliant, to whatever we were doing & we'd move on. But I kept coming up against 'Jekyll & Hyde' - he'd be fine for the most part, then 'suddenly, out of the blue' with something we'd been doing OK on, he'd come 'unglued' again. It took for a friend to point out to me that when he was 'quiet' he was actually holding his breath!! Once I realised the state the poor guy was in & I was adding to it, I was finally able to really
'listen' to him & get us over it.
I always try to work with a horse where he's at at the time, whether I've had him for bit or just got him. Whether he's been 'abused' or pampered. And I don't tend to handle them all that differently, excepting in time frame of doing stuff - whatever they're ready for, they're ready for. If they're not up for it, we don't go further yet. And while many horses I've worked with I don't know their history, agree with Yogi that IME the behaviour of this horse doesn't scream 'abused' to me. Maybe he was, but I wouldn't treat him with kid gloves because of it.