Remember to support your hand while painting and keep it relaxed, the more you try to keep your hands from shaking (and squeeze the paintbrush tightly), the more they will shake. If you can, rest the hand that's holding the pony's head on the desk and try to support the elbow of your paintbrush hand - against the desk, your knee, anything that works for you.
In the end it comes down to practice, though. You can practice painting smooth thin lines on paper and on a curvy piece of plastic - using a paintbrush is really quite different from using a pencil, and acrylic paints don't flow as easily as watercolours, for example. Sometimes it's tricky to find the right consistency so that the paint flows easily but is still opaque. Besides, all brushes are different, you kind of need to familiarize yourself with new or newly trimmed brushes.