Ok I am playing as a paladin and will be hitting lvl 5 soon and will choose my divine bond power(weapon or mount). I have decided to go with the animal companion/mount. Here is quick description of the divine bond power concerning the animal companion.
The second type of bond allows a paladin to gain the service of an unusually intelligent, strong, and loyal steed to serve her in her crusade against evil. This mount is usually a heavy horse (for a Medium paladin) or a pony (for a Small paladin), although more exotic mounts, such as a boar, camel, or dog are also suitable. This mount functions as a druid's animal companion, using the paladin's level as her effective druid level
I am a human and considered medium size. My question is does the animal companion I choose have to be mountable(in my case large sized) right away. The animal companion I have in mind when I get it will be medium size until it hits level 7 using my paladin as the effective druid level then it will become large in size to where I can mount it.
I know then a caviler gets a mount at level 1 and the rules for it are:
A cavalier gains the service of a loyal and trusty steed to carry him into battle. This mount functions as a druid's animal companion, using the cavalier's level as his effective druid level. The creature must be one that he is capable of riding and is suitable as a mount. A Medium cavalier can select a camel or a horse. A Small cavalier can select a pony or wolf, but can also select a boar or a dog if he is at least 4th level. The GM might approve other animals as suitable mounts.
So it has to be mountable right away for a caviler, but the rules for the animal bond for a paladin doesn't specifically say that. So can I chose an animal companion that will eventually become a mount later on after a couple of levels or like the cavalier I have to be able to mount and ride it right away.
To clarify I am trying to get a dragon as a mount. I am using a third party handbook called, Dragon Companion Handbook. It lets you take a dragon as an animal companion and uses a slight variation of the druid companion leveling chart. It has a couple feats you can take along with having a prerequisites: Knowledge (arcana) 4 ranks, animal companion, divine bond (mount), mount, or a nature bond (animal companion) class feature with an effective druid level of 4th: ability to speak draconic. The second feat increases it size to large. I can take the first dragon feat at level 5 and the second at level 7 for my paladin.
It also states that you acquire and advance these dragons as you would an animal companion or mount from the class feature you used as a prerequisite. Also if you multiclass as long as the classes you are using give an effective druid level for an animal companion or mount, they stack.
I have always wanted to be somewhat of a dragon rider and I know there is a third party class for it, but my gm looked at it and said it was too over powered for our campaign. I looked into other alternatives but could not find one that was viable until I found and bought this downloadable guide from Paizo. I showed it to my GM and he said he was fine with it, but I had to stay in the rules of the pathfinder game in order to get it. So hence my above question.