Siblings
To Shukaku, Kurama had always been the perfect older sibling, the one people compared him to.
And oh, how he loathed it.
They claimed the more tails you had, the more power you had (Not true), and that by that logic Kurama was the most powerful. There was no way he was more powerful (in Shukaku's eyes at least) Damn fox couldn't even break the seal keeping him inside the Uzumaki brat. Obviously he was the weak one. Even Shukaku was able to completely take over his Jinchuriki host (by controlling him with insomnia, as Kurama loved to point out).
To Kurama, Shukaku was that bratty, irritating younger sibling who thought that they were hot fucking shit.
To be honest, he found it hilarious.
Winding up Shukaku had long been Kurama's favourite pastime. It wasn't his fault (or so he claimed). The Tanuki just bit so damn hard!
Oh come on, you just know those two would be like this.