None of these are legendaries. Interactable means you must press A while facing it. Some of them have specific requirements.
How to unlock
After having beaten the main game, if you go to the Relic Castle entrance surrounded by "statues", Professor Juniper will show up and give you a RageCandyBar. Upon interacting with one of the "statues", it will reveal itself to be a real Darmanitan. These Darmanitans are quite rare in that they are the only non-Dream World Darmanitans with Zen Mode. They are at Level 35.
Foongus and Amoongus
On Routes 6 and 10, some items on the ground are actually Foongus (both Routes) or Amoongus (Route 10). Interacting with them will cause them to do a small jump and force you into a wild Pokemon battle with them.
After having beaten the main game, a new area of the Dreamyard will be open to you. In the basement of this new area will be a Musharna. It is quite rare in that it is the only non-Dream World Musharna with Telepathy. It only appears on Fridays and it is Level 50.
If you transferred any of the event Raikou, Entei, and Suicune via the Relocator and have any of them in your party, at the dead-end of Lostlorn Forest, the speechless lady will attack you as a Pokemon, making it more of a forced battle than an interaction battle. If the first Pokemon in the ordering of your part is Entei, you will encounter Suicune. If it is Suicune, you will encounter Raikou. And if it is Raikou, you will encounter Entei. Whatever you encounter is treated as Dark-Type. However, if you attack it with a damaging move, it will reveal itself to be Zoroark. It is Level 25.
After completing the main game and going down to the bottom floor of Relic Castle that you were able to access during the main game, you will find that the room has filled up with sand, allowing you to enter the hole near the top of the room. After going through a rather literal maze of rooms, you will see one of the Seven Sages, but what's important is that Volcarona is in the room behind him and it is Level 70, making it the highest-leveled non-boss in the game..