Their still four legged. Think more fell drake less dinosaur. Caller drakes come in non-combat, riding forms (Lesser Callers) and as Greater Caller Drakes (Heavy warhorse). There's also a flying breed but only royalty or generals can get access to those.
What's more interesting is the guild itself. The process to buy a drake requires various papers, stamps and you get your own personal badge dictating which drake belongs to you. Thus if it ever gets stolen, it is easy to find the drake. The guild also likes to know where your drake is. Always. Inns and such don't have stables only guildhouses do. As such you pretty much 'register' your drake whenever you go to a place that has a guild stable. Somehow whenever a drake is stolen it always ends up back in the owners hands. The guild does not take well to anyone stealing what they still consider their property. The Rider or Caller guild is also the premier information guild. They know -everything-. They don't allow anyone else to breed the drakes either. Anyone who has tried mysteriously disappears. It's also rare to see a dead drake, there are some rumors that the guild simply resurrects dead drakes or animates them and paints over the scales to make them look alive.