My favourite animal sidekick/pet/summoning is Kero from Cardcaptor Sakura.
Keroberos is just adorable.
Not only he is adorable, but he’s also a very helpful sidekick, giving Sakura advice on how to use the Clow Cards and how to capture them.
He becomes her friend, as well as her guide and partner.
I used to love the Kero’s Corner he used to do at the end of every episode. A little sad I know, but it was cute.
I also loved how he grew into a full grown lion once they had captured all the four element cards:
And he’s all about the pudding (you can’t hate someone who loves pudding as much as Kero does):