The French Ridouts were Huguenots, Protestants, who fled religious persecution during the late 1600s and settled in the south and east of England at Canterbury, in Kent, and Sherborne, Dorset. There were others with French sounding surnames like Ridou and Ridour. The town of Ridout in Canada is named after Thomas Ridout who emigrated to North America and became prominent in the government of Canada in 1794. Perhaps it is significant that Canada had a large number of French-speaking people.
The English Rideouts (Ridut, Rydhut, Rydhout, Rideway, Ridoutt, etc) go back to the early 1200s in Somerset and Yorkshire with Ridout, Rideout, etc being fairly numerous later in Dorset and Wiltshire. Those authorities which make suggestions for the origin of the surname assume it has a connection with horsemen and was either a nick name or 'some forgotten joke'. Another suggestion is that the first person to use the name lived in a red hut. Still another suggestion is that the spellings ending in 'hout' indicate that the clerk was trying to emphasise a 'ride out' rather than a 'rid ut' pronunciation.
Further support for the horseman origin comes from Heraldry. A John Ridout (or Ridden, Ryden, Royden) of Exeter was granted arms in 1518 which featured 'a Griffin Passant'. Later Ridout/Rideout men were granted Knighthoods and bore arms including 'a White Horse Passant' topped by a wild looking negro head. It was very similar to those of John Ridout. In each case the motto was 'Tout Toit Chevalier' which means 'Always a Knight'. The motto is said to be a play on words for the surname Rid(e)out which is said to derive from Knight or mounted rider.