sheep & horses are no real problem if they move out of the way!
In case of an engine failure your aircraft has just gone completely quiet apart from some wind noise. And even if the animals see you coming, they will probably not recognise you for what danger you are and move out of the way. So I would not bet on animals conveniently creating a nice clear landing strip for you in your chosen field.
So in that respect any type of livestock in the field is bad news, unless you manage to avoid them.
Having said that (but now I'm a glider pilot talking), if you have the choice, sheep and horses are better (less bad) than cows and ostriches, indeed. Not because of the damage during the landing roll, but because cows and ostriches are very inquisitive and will damage the aircraft while trying to investigate what just landed in "their" field. Horses and sheep are apparently less inquisitive, or are more easily shooed away. At least, that's what I've been told - I have yet to make my first outlanding.