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.
Focus on this. Cavalier mount ability is very specific on what the cavalier can take as his mount. An eidolon may look like a horse, camel, dog, boar, etc. but is not one and despite a selection of abilities that mimic such an animal it will never be one. Thus it defaults to DM approval, which, as a DM, I would grant to higher level characters, but that's a different story all together.
