The ponies' reactions would vary greatly. Rarity might be fascinated by Optimus Prime's design and might even try to make some sort of fashionable accessory inspired by him. Applejack would be more practical and would wonder what he was doing in Equestria and if he had any good intentions. Optimus would see the ponies as a new and interesting species. He would be patient with their initial wariness of him and would try to show them that he means no harm. He might also be a bit taken aback by their colorful and somewhat chaotic world compared to the more structured world of the Autobots.
The characters in One Piece and Sonic are very different. Sonic is a heroic hedgehog from a modern - ish world, while the One Piece characters are pirates, marines, and various other fantastical beings. When they meet, there could be a lot of misunderstandings at first. For example, Sonic might not understand the concept of the Pirate King right away. And the One Piece characters might be freaked out by Sonic's non - human appearance. But as they get to know each other, they would find common ground. Sonic could help the Straw Hat Pirates in their adventures with his speed, and the pirates could introduce Sonic to the wonders of their world, like the different islands and the secrets of the Grand Line.
Well, there are also those characters who might be a bit shocked or confused. Take Levi from Attack on Titan. He's a very serious and no - nonsense type of character. If he were to read some of the fanfic about him that deviated too much from his own self - perception, like being made into a comical or overly emotional character in the fanfic, he might be taken aback at first. But then he might also realize it's just a different interpretation and not take it too seriously.