Donald Ross' baptism is on ScotandPeople if you look under Church Register - Old parish Register's - Births. It is indexed under Donald, but I searched as D (beginning with - in search options). If it says he is Natural then he was illegitimate. But under Scot's Law if the parents marry after he becomes legitimate. IF he was illegitimate there will probably be an entry in the Kirk Session Minutes about fornication - unfortunately they are only available in the Registers in Edinburgh or a couple of other centers around Scotland, not online yet, as Ann has already said. The KSM are fascinating reading, but I usually build up a list to investigate then go down and spend a day checking them while visiting friends.
I have found a Janet Ross born Easter Ross dates 7/9/1815 with a strange entry under the parents - ALEXR. ROSS/ANNE ROSS FR 163 MONTH NOT STATED IN ENTRY (FR163) - I can't remember without checking if those are the parents you are looking for, but that extra makes me wonder if the baptism was allot later than the birth. Anyone else seen that before?
I always search 2-5 years either side of the date of the birth. I have one that was baptised many years after birth - think the mother suddenly got religion and got her two legitimate children baptised on the same day when they were in their teens. So it pays to widen searches and be flexible on spellings and if it is indexed by something like Don'd or Donald even when you know what you are looking for.