It's clearly the totally innocent Lyra Horn-Warmer™ Plushie, that is totally innocent.

Twilight wore it to bed each night in the past, when the air would get chilly. Alas, this winter it was not meant to be...

With Twilight growing into her new alicorn form, the Lyra plushie just simply can not take her length or girth anymore.

...

I was talking about the length or girth of her horn. What else would I be talking about?