Yes it was quite common to use horse hair to bind the plaster. I found it in our walls as well. We didn't find anything of interest but on the front rafter in this bedroom it was written "front rafter rear dormer" the guy that cut it probably put that on there for the poor chump that had to climb up there and nail it. we also found some newspapaper from 1980 in the ceiling blocking an opening so the insulation wouldn't fall through when they blew it in and a piece of styrofoam around a pipe along the oustide wall to keep it from freezing and I don't know how it got there unless the guy that built our kitchen in 1999 put it in there from the bottom when they were doing that.