Sod's Law says it was going to be a white (OK - grey) horse for your first assignment.
One of my early assignments was a series of pictures of a black Porsche and I forgot to allow extra exposure for that in the buzz at the time. The customer was happy enough, but the shots could have been better.
It's a nice shot mate, and thankfully, customers aren't as critical of our stuff as we are ourselves - I'm sure she'll be pleased with it and you've nothing to reproach yourself for. You're just tending towards over exposure around the top of the head there and you've done a good job with the processing. If you could just bring out the contrast a little between that bit of white mane falling forwards from between the ears and the underlying grey forehead it would be spot on.
You've got the angle much the same as I'd have done, slightly below the horse's eye level, but as this a close up portrait shot I'd have gone for a completely none distracting and as plain a background as possible , either totally against the sky or totally against the green foliage and as oof as possible - not for a combination of the two.
You've left sufficent space to the right of the horse for good composition, but as Neos mentioned that same space looks a little empty towards the bottom of the pic. If you could have moved to the right and got the horse's shoulder in shot with his head turning to look at you, you'd have had perfect compostion, but hey! this is working with animals! It can be very difficult if not nigh on imposssble
The more you do this stuff the more you realise that background is everything, and you become very aware of distractions like that building on the right when you're framing your shots.
I've only been this critical because I know it's what you want mate. It's a good first job well done and I'm sure the customer will be very pleased with it! :thumb: