Well, you could use names like 'Viper'. A viper is a venomous snake, so it gives the idea of someone who is dangerous and sneaky. 'Blade' is also a good choice. It makes you think of something sharp and menacing, just like a kidnapper might be. Then there's 'Lockjaw', which gives the impression of someone who is unyielding and cruel.
There could also be a group of villains working together to kidnap Sonic. For instance, a combination of Eggman and some of his more powerful robot creations along with a couple of other villains from Sonic's rogues gallery. They might have a grand plan that requires Sonic's capture, like using his power to open a portal to a world full of chaos energy that they can then control.
A jealous rival of Inuyasha might kidnap Kagome. For example, another demon who wants Inuyasha for himself might think that by taking Kagome, he can get Inuyasha to do his bidding. This would create a lot of drama and conflict between Inuyasha and the kidnapper.
The Grimm could also be involved in an unusual way. Perhaps a new type of Grimm that has the intelligence to kidnap. They might be working with a human or Faunus villain who has some sort of control over them, and they take the character to an unknown location where the RWBY team has to face not only the Grimm but also the mastermind behind it.
Maybe a group of outcasts from outside the walls. They could have heard of Mikasa's strength and think that by kidnapping her, they can gain some sort of leverage over the people within the walls. They might be desperate for resources or a place to live and believe that Mikasa is their ticket to getting what they want.
The Joker is a likely kidnapper. He has a history of causing chaos and targeting the Bat - family. He might see kidnapping Batgirl as a way to get back at Batman or just for his own twisted amusement.
In certain fanfictions, a jealous rival of Amy Rose could be the kidnapper. This could be another female character who has feelings for Sonic and sees Amy Rose as a threat. She might think that by getting rid of Amy Rose, she can have Sonic all to herself. Or it could be a character from Amy Rose's past who has some unresolved issues with her and decides to kidnap her to get revenge or make her pay for something that happened a long time ago.
It could be the Separatists. They might see Anakin as a valuable hostage to use against the Jedi and the Republic. His skills as a Jedi Knight make him a prime target for them to gain some sort of leverage in the war.
It could be a jealous rival of the spirit animals' owners. This person might be envious of the special bond and powers that come with having a spirit animal and decides to kidnap them to spite the owners.