Horses
Here are some suggestions to make your job easier...if that's possible when you own your own website ... mmm
Age - Do they age. Do they retire. If so at what rate? Are there restrictions to what a 2 year old can do. Try to keep the game realistic
Breeds - try to allow all breeds in your game (this makes for variety) unless you have a thoroughbred racing game or something, but also consider will you allow Shetlands to show jump?? You may have to place restriction on that sort of thing ... also don't name all of the breeds, you will no doubt miss out a couple of thousand breeds and offend at least fifty people
Colour - Once again don't put yourself in a trap of limiting the colours, photos are also really good to identify colours
Naming - perhaps each breeder or person must put a suffix in front of the name to identify each horse as theirs or one that they have breed. i.e. A suffix I use in games is DIA so I might have DIA Cavlicade IX
Health - Must they visit a farrier or a vet? Do they get sick or can they get overworked?
Feeding - Do horses need to be fed. Must the food by bought? Could someone grow their own food? Whose responsibility is it to feed the horse? (Owner or trainer or stable owner)
Breeding - How many times can a stallion mate? How many foals can a mare have? Breed regulations (Can a Hanoverian and a Thoroughbred have a Appaloosa foal?). Age restrictions on breeding. A restriction on how many foals one player can have? (Do they need a breeding permit for more than one foal a week)
Events - do they have to be appropriate (I know I am repeating myself here, but it is important)