Yes, any breed of horse or pony can get PPID, and there is no gender predilection.

Risk increases with age, and clinical signs are most frequently first seen in horses in their mid-teens.

However data collected from over 3100 horses tested for PPID during Boehringer Ingelheim's Talk about Laminitis campaign in spring 2012 have suggested that PPID could be common in laminitic horses as young as 10 years old - Veterinary Record 2012;171:239

Horses as young as 7 (Heinrichs et al 1990, Orth et al 1982) have been diagnosed with PPID.
Donaldson et al. 2004 gives the age range of horses suspected of having PPID as 3 to 28 years, with median age 15.5 (however some of these could have been falsely diagnosed in the autumn as seasonal reference ranges for ACTH were not used).

There may be differences between horses and ponies.  At the 2011 BEVA Congress Andy Durham suggested that PPID may be a cause of IR in horses.  Ponies develop PPID at a younger age than horses, suggesting that IR and hyperinsulinaemia may predispose ponies to PPID.  In 744 cases of PPID studied, insulin was higher in younger PPID ponies than in older PPID ponies, suggesting that they were IR at the onset of PPID, but that weight loss which is frequently a symptom of PPID improved their insulin sensitivity.

Nicholas Frank suggests that PPID should be considered in any horse older than 15, but that ponies with EMS can develop PPID at a younger age (> 10) - ponies that become obese at a young age and then develop Equine Metabolic Syndrome have an increased risk of developing PPID when they reach their teens - EMS appears to predispose a pony to PPID.

It should be remembered that testing for PPID has improved significantly in recent years and that it has only recently been recognised that the majority (c. 90%) of laminitis cases have an endocrine cause.  

We've recently heard several reports of horses with PPID showing aggressive and/or excitable behaviour.

One horse was reported as changing from being a gentle giant to kicking out (particularly when having his belly groomed or rugs changed), biting, squealing, being sensitive to touch and difficult to catch, with his owner being unable to pick up his feet or groom him some days.  The horse had recently started on Prascend but the unusual behaviour preceded treatment and had been going on for several months.  Some days he seemed fine but lethargic, others horrible.  He was also difficult to keep weight on and had had several weeks treatment for possible gastric ulcers with Gastroguard - scoping post treatment found no evidence of gastric ulcers, he wasn't scoped before treatment started so it isn't known whether he did actually have ulcers.

Another owner reported a similar experience with her pony gelding.  Before starting Prascend he wouldn't tolerate being brushed and would lift a leg in warning if his owner for example looked at his sheath area (which was swollen).  Prascend was started and he became aggressive towards other horses so had to be kept separate, he was unpredictable and although he appeared to enjoy being ridden, he would sometimes stop and refuse to go forwards without a battle.  After several months of treatment with Prascend he's back to his normal self, and his owner concluded that his hormone levels had to settle down and adjust.

A cob gelding in his 20s was diagnosed with PPID and started treatment with Prascend just over a year ago.  He became aggressive after starting treatment, kicking out when being handled and biting, becoming very sensitive to being touched, nervous and difficult to catch.  He has arthritis in front and hind legs.

We do rarely hear reports of hyper-excitability, nervousness and behaviour changes with both PPID and pergolide/Prascend, but it seems to be far more normal for horses with PPID to be lethargic and docile.  What may happen is as the Prascend takes effect, the anti-inflammatory, pain relieving and sedating hormones (e.g. alpha-MSH, beta-endorphin) produced in excess are reduced, and any existing pain is likely to be more obvious - this is often seen if horses have arthritis.  

In the Ask the vet live: Equine Cushing’s Disease (PPID) in September 2012 with Nicholas Frank and Marian Little, an owner asked:
Q. Can treatment with pergolide make horses more irritable/unpleasant?
And received the answer:

Owners of PPID horses often say “the lights are on but no one’s home” – over time PPID tends to
make horses more lethargic & docile.
Often when treated with pergolide horses become much more energetic and perhaps bolshy – likely
restoring the normal function of the pituitary that the horse lost some time ago – horses with
advanced PPID are often very gentle with sweet personalities, and sometimes owners prefer their
PPID horse to the one they find after treatment!
Also when some of the excess “feel-good” hormones are removed with treatment, some horses start
to feel age-related aches & pains that were masked with PPID – need to rule these out. 

We know that horses can have large variations in hormone levels just a few days (or hours) apart, perhaps that might help explain daily variations in temperament.

Aggressive or unusual behaviour could be a pain reaction, and may not necessarily be linked to the PPID or treatment, and your vet should always be consulted.

The FDA New Animal Drug Application for Prascend Sept 2011 lists " Behaviour change – 5% - included kicking, aggression, agitation, nervous behaviour and increased activity.  One horse required a temporary reduction in dose due to energetic behaviour" when horses with PPID were started on 1 mg Prascend. 

If you believe that aggressive behaviour is related to Prascend, you should report it: 
Please report adverse reactions to Prascend
But be certain that the changes seen started after treatment with Prascend was introduced.

Ensure that the horse's diet is providing all essential nutrients in the correct balance - Diet.