I know I already posted this, but I think something went wonky, so I'm trying again...
I flopped around a lot when pondering this prompt, but, in the end, I'm going with Shooter from Durarara!! Shooter is Celty's horse/bike. Though his classic form is a headless horse, in the show, he usually takes the form of a black motorbike to blend in with Ikebukuro and can transform into just about any form. He doesn't show it often, but I love his personality, especially his relationship with Shizuo!
This was late in the first place, so I managed to narrow this one down to only a few in the hopes of saving time!
Kuro - Blue Exorcist
Kuro was Shiro's familiar. When he died, Kuro waited patiently for him, and everyone was afraid to break it to him that he wouldn't be returning, fearing that the demon would turn on them without his master, which is exactly what happened. Through their mutual loss, Rin was able to reach him before he had to be taken out, so Rin ended up kind of inheriting his adoptive father's familiar. I love the bond that Rin and Kuro share; they're so cute together!
Akamaru - Naruto
Akamaru and Pakkun were super close in the running, but I think Akamaru wins out. Kiba's better half, this ninja hound is adorable and badass. In the original series, he was a little puppy that Kiba would often carry around on his head or in his jacket. By Shippuden, he's full grown, and he's massive, but he still kinda acts like a puppy, just like my dog! (We call him Tweeter Pan because when he whines, he sounds like a tweety bird, and he's a 12 year old puppy!)
As always, hope you enjoyed! If you wanna learn more about me/ be my friend/ be tagged in these, please don't hesitate to ask! I'm nice, I swear (mostly ;p)! Or I guess you can follow my 100 Days of Anime collection, or both if you feel so inclined!
Until next time!