I read The Art of Voice Acting and there was a part about exercises before doing voice acting.
He mentioned yawning was good for voice acting because then your mouth is relaxed.
There are lots of other exercises he mentioned for your legs, arms, body, neck, face and even your tongue and lips.
The pages you find these exercises are pages 28-33.
The exercise for your horse lips is you just take a deep breath then release the air through your lips. Let your lips "flutter" as you breath out.
If you are interested in voice acting The Art of Voice Acting is a perfect book to start Learning.