Yes, correct, you can buy a hippogriff. You can also buy an elephant, a leopard, a griffon, an oliphant and a worg. Assuming these things are available. And they sometimes are, as evidenced by the griffon and hippogriff both being available on this generation.
There are a few considerations. With some animals, the question is less, "can it be bought" and more, "can it be controlled." The difficulty is that while your first level ranger might be able to buy a griffon, the likelihood is that your first level ranger, upon approaching the griffon, will fail the necessary roll to dominate the animal and be immediately ripped into. Following that failure, should it occur, the ranger will have to progress to a higher level before attempting to control that particular species of animal again.
This roll is simple. The fighter-class character rolls a d6 and adds his or her level. The griffon rolls a d6 and adds its hit dice. If the fighter's total is higher, the griffon will allow itself to be ridden. Certain modifiers can be added to the fighter's total if the fighter has a husbandry background (+1), if the animal can be spoken to (charisma bonus), and if the fighter has controlled an animal of its nature before (+1).
Non-fighter characters cannot control the griffon, ever. A spellcaster or monk could learn to speak with animals and gain the griffon's trust, and the griffon may allow the spellcaster to ride it, but this would be a 'favor,' and not the castor controlling the animal. In any case, the animal would not allow itself to be ridden into combat by anything that did not control it.
Incidentally, once a fighter-class proves themselves able to control an animal once does not mean that every animal of that type can be controlled by that fighter; the roll must always be made with every new animal. However, if the fighter has been successful previously, the fighter no longer has to be a new level to control other animals of that type ... but must reach a new level to attempt to control a particular animal that the fighter failed with previously.
A second consideration is that even if the animal is controlled, the morale of the animal at the beginning is quite poor. Animals typically start with a 9 morale, which means that a 9 or better must be rolled on 2d6 before the animal will allow itself to enter into combat. In addition, this morale must be rolled again if the animal is 'stunned' at any time during the combat (that is, if more than one quarter of its hit points are taken in the space of one round). If it fails morale, it will avoid combat. If this happens on account of being wounded, the animal will flee, taking its rider with it; the rider must then risk jumping from the animal while it flees, or take 3-12 rounds (minus rider's level) to calm the animal long enough for the rider to turn the animal around, return to within 60 feet of the combat and dismount. So it can be problematic for the rider.
However, every time the animal succeeds in making a morale check, the morale drops 1 point; so if it made its morale check to enter it's first combat, it would need to roll an 8 or better to stay with the combat after it was stunned. If the second roll was made, the animal's morale would drop to 7 and the animal would remain in the combat until the rider retreated (in other words, the animal would fight until death, by virtue of its training).
The lowest morale an animal can have is 3.
The untrained animal is a bigger problem. It must be first trained by some individual who can control it; after this, it must remain in the company of its trainer for one month per hit die, during which time it must be controlled decently within a structure designed to contain it ... whereupon it will develop an attachment to its trainer and have a morale of 12. Usually, mock fights are staged to lower the animal's morale to 9, forcing it into apparent combats until it gains the additional 3 morale improvement. Thereupon it can be sold to others.
Other animals, those which can be naturally domesticated, like the war dog or the elephant, do not require a roll to determine if they can be controlled. Oliphants fit into this category. Morale remains a factor, and wild animals must still be trained as described above (with the control roll included).
It will be noticed that the price of the animals is low. This is because the third problem is usually housing and feeding the animals, which can be horrendously expensive. So yes, you can buy an oliphant, provided you can feed it.
Animals which are obtained as kittens or as eggs can be 'controlled' from the outset ... but the time factor in waiting for the animal to reach maturity, and thus its initial 12 morale, may be extended to longer than one month per hit die. I take note that the worg doesn't have a price for a young pup. I'm sure this can be worked out by comparing the price of a baby leopard to one that is full grown.
Oh, the leopard isn't an 'attack animal' ... but if it views its master being attacked, it will defend. It might bear mentioning that if its master isn't around, it is likely to attack any individual unable to 'control' it ... which would mean in this case not bringing it to heel, but having it keep its distance. This might apply to many of these animals, but it could not be done if the animal's master was present.
Next: the Perfumer's Shop.