The Croesus' first appeared on Howrse in the spring of 2010. To get one, players had to be in the first 100 players to reach level 10 of the 'old' Achievements. Myself and LucyLou were lucky enough to be amongst those first 100 players. As it happened, about 120 Croesus' were given out originally, due to variations in people getting their level 10 of the Achievements.
Each of these first 100 or so players was also awarded a 'VIP' icon which remains on their profile pages. After the first 100+ players had won their Croesus', Howrse decided to allow people the chance of winning one (without the 'VIP' badge icon!) in Horns of Plenty. This obviously meant many players were buying a lot of passes in the hope of winning one of these rare Divine horses.
Now all the Croesus' have been given out and there are less than two hundred in the game - making them both rare and valuable. One or two have been sold in the auctions and have fetched a huge amount of equus - around 4.5 million!
So, what are Croesus' and what do they do? Well here is Howrse's own explanation:
Croesus is a Divine horse that produces money instead of manure. Every 13th day of each month, Croesus earns his owner 1 pass provided the owner was logged on the previous day. If the 13th falls on a Friday, Croesus earns his owner 2 passes.
Every month, one horse among those named Junior Croesus is turned into Croesus.
Aside from this, Croesus' skills aren't that great - just 470 skills when you get them. They can be Clouded so will make around 505 skills. Like all Divine's - they are male and cannot breed.
However, they are a lovely addition to anyone's collection - especially if you also have that little pink 'VIP icon sitting on your page!