You need to age her up

1. go to their public page, where you can give apples (this is true for both your horse or other people's horses. beware, if you give Midas touch to another player's donkey, you won't get to use it on yours that month.

2. you can't give it to an underage donkey as horses under 18 months cannot receive those customization items.

3. it replenishes on the first of the month but you can use it any day. Use it or lose it, for example if you don't use April's by the 30th, for may you will get may's but you won't have April's saved up.

On Classic, mouse over the Donkey's picture, and you'll see icons appear towards the bottom of that area -- click those to use the Pack.

I do not know how it works on mobile/app as I never use it.

On Classic, when you mouse over the image, above you'll see a set of icons, and also below:

The ones above are (left to right) to make the horse's image larger or smaller, to enlarge/reduce the backdrop, to flip the horse horizontally, to get the link for the public page for that horse, to download the picture.

Below, L to R, to use the Animations feature, to use the Golden Apple, to use the Helios' Ray (mine shows up as the Unlimited one, without VIP I think it shows up as the regular one), and to use the Apollo's Lyre.

NB: If your horse does *not* have a Harmony Pack, you can still use those icons/buttons, it's just that you'll have to pay for the item (except Animations) or use one from inventory. If your horse has a Harmony Pack, it will just say on the page about "Included with your Pack".

Thanks for the answers!