Linseed (Linum usitatissimum) and its by-products are common supplements used in equine diets and are claimed to have beneficial health effects. In this study, the effect of linseed groat-based fibrous feed supplements on diet digestibility was studied. Also, possible detrimental health effects due to continuous feeding of supplemental feeds containing linseed were examined by evaluating blood parameters. The experimental design was arranged as two balanced 3 × 3 Latin Squares. The horses were individually fed at the maintenance energy level, the forage-to-concentrate ratio being 70:30, with three diets: (A) Control diet consisting of dried hay and whole oats; (B) Control diet + Feed 1; and (C) Control diet + Feed 2. Feed 1 contained 70% of linseed groats, 15% dried carrot, 10% dried garlic and 5% molasses. Feed 2 contained 65% linseed groats, 15% molassed sugar-beet pulp, 10% dried garlic, 5% dried carrot and 5% molasses. Digestibility data were obtained by using chromium mordanted straw as an indigestible external marker for the estimation of apparent digestibility. Blood samples were collected from the jugular vein at the end of each feeding period to evaluate the possible effects of the supplemented diets B and C on the health of the horses. Diets B and C had a higher digestibility of crude protein compared to the control diet A (p < 0.05). In addition, the digestibility of ether extract was higher in the supplemented diets than in the basal feeding (p < 0.01). There were no statistically significant differences or trends (p > 0.05) in the blood parameters between the treatments. It is concluded that linseed groat-based supplements (offering approximately 6.3%-6.7% linseed groats in the diet's dry matter (DM), or 0.8 g/kg BW/d), and feed containing soluble fibre sources (sugar-beet pulp, dehydrated carrot), improved the crude protein and fat digestibility of hay-oats diets of horses, and can be used, for example, in feeding strategies replacing grains in the horse rations in order to reduce the intake of starch without any adverse effects on the blood parameters and health of the horses.