Are there any rare magical objects that Harry has never seen?

Question

In the Harry Potter series, Harry always gets his hands on the rarest and unique magical items of the wizarding world:

• He saw the Mirror of Erised,
• He found the Philosopher's Stone,
• The Sword of Gryffindor appears to him,
• He owns the Marauder's Map,
• He used a Time-Turner with the help of Hermione (we know that many Time-Turners exist, but it still rare),
• He saw the Veil in the Ministry of Magic
• He benefits from the use of the Deluminator, which is possessed by Ron,
• He took the eye of Mad-Eye Moody (and buried it under a tree, but still),
• He possessed all the Deathly Hallows and keeps one (the cloak),
• He found the Horcruxes, and some of them were precious relics,
• ...

Is there any rare and powerful magical object that Harry has never seen, but exists in the series' lore ?

Answer 1

Harry would never have seen Salazar Slytherin’s wand.

Salazar Slytherin’s wand is one rare magical object that Harry wouldn’t have interacted with. We know from the JKR Pottermore writing on Ilvermorny that Salazar Slytherin’s wand had been passed down for centuries to members of his family, and that his wand had a basilisk horn core (which would be a quite rare wand core since basilisks were rare and dangerous creatures).

In all the years that she had lived with it, Isolt had never known that she held in her hand the wand of Salazar Slytherin, one of the founders of Hogwarts, and that it contained a fragment of a magical snake’s horn: in this case, a Basilisk. The wand had been taught by its creator to ‘sleep’ when so instructed, and this secret had been handed down through the centuries to each member of Slytherin’s family who possessed it.
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During what would be sometime in the seventeenth century (Isolt was born in 1603 and Ilvermorny was founded in the seventeenth century), Isolt buried this wand outside the grounds of Ilvermorny.

Slytherin’s wand remained inactive following Gormlaith’s command in Parseltongue. Isolt could not speak the language, but, in any case, she no longer wanted to touch the wand that was the last relic of her unhappy childhood. She and James buried it outside the grounds.
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It likely didn’t stay as a wand underground - a tree grew from where it was buried, and the implication seems to be that the wand itself somehow sprouted into this unusual tree.

Within a year an unknown species of snakewood tree had grown out of the earth on the spot where the wand was buried. It resisted all attempts to prune or kill it, but after several years the leaves were found to contain powerful medicinal properties.
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Since this wand had been buried and became a tree hundreds of years before Harry was born, (and of course we never see Harry have anything to do with it in the seven books), it’s a reasonable conclusion that Harry would have never have seen or interacted with Salazar Slytherin’s wand. By the time he was born, it’s been a tree for centuries - the wand no longer exists as a wand.

Answer 2

This question cannot be answered as it is attempting to provide proof of a negative.

We know from non-Potter related material (ie: Fantastic Beasts) that there is, for instance, a magical trunk which provides a large pocket of extra-dimensional space.

But do we know whether Harry has definitively never seen this? Of course not.

To answer this question, there would first need to be a definitive 100% complete list of all "rare" magical items in existence or ever in existence (after all, time travel is possible and used in two different Potter stories), and then with that list we'd need definitive answers confirming that Potter had never seen them.