I just remembered a milk based glue called casein. I recall it gets really hard. Im gonna buy some horse hair for experiment. Seems to me, being hair is a protein, a protein based glue like casein would bind it.
Im thinking to soak the hair for a period of time, then put the strands in a welded metal box, then compress it with a hydraulic press until dry. I want to match as close as i can the density of horn. Then use the density ratio of hair to horn, and build a bow with that percentage in mind, like say its 75% of horn density, i would build the bow 25% larger than a horn bow of equal weight.