Chayenne's Magical Key: A very rare key today, as it could only be obtained from the creature "Chayenne" during Tibia's 15-year festivities.
Key 0021: This item cannot be traded by the Market. This key is a relic of Tibia's past, as far as we know, it is only found in the hands of collectors.
Key 0020: Also known as "Jail key". The key once opened the cells of Thais Prison. But, with update 8.0, the key stopped working. Spawn every update in the chain. Only available in Antica.
Key 0005: This is an old key from the city of Thais. It is a relic of Tibia's past, and as far as we know it is only found in the hands of collectors.
Key 0004: as far as we know it is only found in the hands of collectors.
Key 0000: I used to open all the doors in Tibia. However, this is no longer the case. Shapeshifter owned this key and many other unique keys. Since Shapeshifter was hacked, his main collection (including the 0000 key) has been lost. Other collectors located the hacker and bought a collection of keys to preserve his history and did so successfully. This key was added to the game by one of the map's creators (before CipSoft became a large company they used volunteers to make areas). That person secretly added a key to the game. When CipSoft discovered that there was a hidden key, they removed its function.
In my opinion, these are the ones that stand out among the others in the "rare" question, but there are others, but out of the game only in the memories.
I will leave you a related question. https://www.tibiadown.com/respondeu-quais-sao-todas-as-chaves-sem-numero-em-seu-nome-e-para-que-servem/