How was pony last night? Her stall a mess again? Is she close to coming in heat? As everyone else said, the excess salt licking would make her drink more, and urinate more. From previous pic of Cyn that you posted, she is such a curious horse, and would need something to keep her occupied while stalled. We have some of these in the barn.
Also, they have fillers for toys, that the horse can lick.
Mabe Pony needs a Jolly Ball in her stall, like what Afleet Alex has in his. He seems to enjoy it a lot.
A mouse fell in her bucket last night and drowned, so she didn't drink out of it at least half the night because of that, so it wasn't a real good test. See you all later, I'm on the road again today, back late this afternoon.