All [whoever it is]'s hopes are pinned on this horse – and when saddled up, he benefits from the trusting bond he has forged with his rider, too.

I'm no horsewoman - but this might give you a starting-point to play around with.

Is there a particular term that's troubling you here, @Sebastian? There's lots of different ways you could approach this one - and any one of the components (Hoffnungstraeger / vertrauensvolle Bindung / unter dem Sattel) could really be a KudoZ question in its own right!