That moment when you want Nanase to have a spell that can give someone fox ears and a tail, but you don't have a GREAT reason for for her to get that spell, but Ellen likes to act like a cat sometimes, so...

That's what you go with.

Nanase wanted to give Ellen cat ears and a tail enough that she got a spell for it.

And also she can make fox ears and fox tails if she wants, and presumably other animals.

Because REASONS.

(Okay, the actual reason would be that spells might be more difficult without magic helping more, but they are more flexible. Plus Nanase could have liked the idea of herself having fox ears and a tail, so there's a lot of wiggle room there. Everything adds up, this totally makes sense, no further questions.)