2004 April Fools
The 2004 April Fools' Day event/joke was the first joke of its kind released on 1 April 2004. It involved the arrival of Diango to the game. Jagex claimed that Diango was a cowboy selling horses (as there was popular demand for horses) but in actual fact he was selling toy horsies. The player could buy them for 150 coins.
Diango still sells them in his toy store, and the item is now referred to as a toy horsey. They were also later obtainable as part of the Construction skill, being able to be made using a plank and a crafting surface in a player-owned house.