I've had references to why Angie grew horns and how it's seen in Phinora culture here and there throughout various comics. Pretty much, it's looked down upon because it's a sign of weakness. A Phinora grows horns when they're so inexperienced with transforming into their other form, they mess up their transformation and get stuck in the very first phase of it. This often happens when they try to transform when they're experiencing intense emotions.
This happened to Angie in the Scarlett battle. And as you can see here, it happened to Cora when Hajii got badly injured on a hunt. (for those that forgot, Hajii is Angie's daddy) Despite growing up on the island, Cora always slacked off when it came to training in her other form and she paid for it.
On a side note, I wish I'd actually drawn this at a hi resolution. I was sketching and liked it and didn't think to ink it at a larger size.