Hmm... "Equipped creature has hexproof.
Whenever equipped creature attacks or blocks, the next instant spell you cast this turn costs less to cast."
That's a pretty hefty equip cost. Is the benefit really THAT impressive?

Well.... hmm.

-1 mana (and no loss of tempo, as you can already have used it) for +3 mana; so a net +2 - assuming you are using instants each turn and a creature that can safely attack. (And if you have vigilant creatures it's even better you can get the benefit on opponents turns too)

I can see a lot of decks wanting a 2-mana boost.

The risk is that it goes away once your creature is killed. Which is a huge risk; greatly mitigated by the hexproof.

So looked at this way; it's probably surprisingly fair. But it's a big feel-bad downside cost.

Looked at another way? This is hexproof, on demand, for any creature; at instant speed. The only downside? It costs a resource I don't NEED any more; my threats are on the table. All I might need is instants - and hey! It makes THOSE cheaper too!

So yeah - this is a specialised card, but I can see it. I don't think mythic though - that downside is just too nasty for it to have the splash factor.

(Equip is sorcery speed, so you can't grant hexproof in response to removal with this card on its own. You'd need a Leonin Shikari or an Auriok Windwalker.)

Oh, damn. Ok; that's less good then.

The windwalker is another interesting idea though - removing the equip costs altogether makes the discount pretty mighty.

As unpopular as it is I've decided to keep the equip cost from a flavor standpoint.