When in a group such as a smob or PK group, type "group" and then count up from the bottom of the list. Your horse is the number you get to when you reach your name. E.g., If your name is Bob, in the following group your horse would be 4.horse, e.g., ride 4.horse, dism 4.horse, etc.
Your group consists of:
Ann (Head of group)
This group horse numbering can sometimes by incorrect if a new member joins, you regroup, someone in the group keeps riding the wrong horse, or someone loses their horse, but for the most part if everyone rides their horse number it will all go smoothly.
Alternatively, you can buy a horse at a Pet shop as then you can name it when you buy it, "buy warhorse lightning" for example. Now you can "ride lightning", etc., while with a group for easier horse targeting.
Tip: do a whois <name> first to make sure the name you want to name your horse isn't already taken by a player as otherwise your horse won't have a name.