Let’s start by switching to the Calligraphic pen tool and setting a Width of 0.05 and a Thinning of 0.2 in the tool’s controls bar (14.3 The Calligraphic Pen Tool). If you have a graphics tablet, enable the pen pressure for varying the stroke width (14.3.1.1 Pressure Sensitivity). The Angle and Fixation parameters only make sense for calligraphy, not freehand drawing, so we’ll set the Fixation to 0, which effectively turns a fixed-angle pen into a round brush that has no orientation (14.3.2 Angle).
I’ve never drawn a horse before, and my first several strokes make this brutally obvious. With a beginning like this, I think many would be tempted, as I was, to give up right then and there. But patience and hard work pay off, especially in Inkscape. Just keep throwing strokes onto the canvas, undoing, tweaking, and throwing again. Sooner or later, something which is not entirely dreadful will flash through the tangle of bad lines.
For most people, drawing objects requires visual aids; you may find it hard to visualize on your own how a horse’s body curves or which way the legs bend. I found that photos of real horses were of little help to me. Much more inspirational were stylized drawings of horses, where other, more capable artists have already done the hard work of abstracting and amplifying the core equestrian visual features. A Google image search will provide you with plenty of reference material. After much sketching, undoing, and emphasizing (by adding pen pressure), I arrived at this sketch:
