Large Nature Oracle, Bonded Mount Revelation...

[karossii]

Bonded Mount (Su): You gain the service of an unusually intelligent, strong, and loyal mount. The creature must be one that you are capable of riding and is suitable as a mount. A Medium oracle can select a camel or a horse. A Small oracle can select a pony or wolf, but can also select a boar or a dog if she is at least 4th level. This mount functions as a druid’s animal companion, using your oracle level as your effective druid level. Bonded mounts have an Intelligence score of at least 6.

So if I am a large race, what are the RAW available options?

Assuming it grants me a Huge mount, how would that interact later on in the character's career, if the Mammoth Rider PrC were taken?

[Caderyn]

Because there are no core large races its not really a rules question, at this stage you have moved into the realm of homebrew at which time it becomes the GM's job to decide what is a valid mount for you.

[Captain Zoom]

What Caderyn said, but I'll add that you and your GM should concentrate on "The creature must be one that you are capable of riding and is suitable as a mount."

Some GMs don't treat the listed mounts as a restrictive list, but rather an illustrative one. I don't want to get into a debate over which is right, and really don't care, but discuss it with your GM and if the two of you can agree on a mount, I'd say you're golden.

[karossii]

Actually, the rules allow for large races via the Advanced Race Guide and Race Builder. Those are specifically rules...

Aside from that, I cannot find the rules/guidelines on size of mount vs. size of rider. I have heard that a mount must be a size category larger. I have also heard some mounts can be the same size as the rider. This would be applicable regardless of the race's natural size.