Eidolons as mounts

[Xiviar]

Is there any way to use a summoner's eidolon as a mount before reaching Summoner Level 8th (required for large size), other than playing as a small race or via reduce person spell?

[Artanthos]

Baring playing a small race or using Enlarge Person/Reduce Person, no.

[Gilfalas]

Is the Mount Evolution REQUIRED to use the Eidolon as a mount? I would think yes since it exists but since the Eidolon starts out as a very intelligent creature and it does not use tricks or training I cannot see any rules reason it cannot.

What am I missing?

[Starglim]

Eidolons: Evolutions wrote:

Mount (Ex): An eidolon is properly skilled and formed to serve as a combat-trained mount. The eidolon must be at least one size category larger than its rider.

One size larger is not an absolute Pathfinder rule, but is explicitly required for the Mount evolution to be effective.

Gilfalas wrote:

Is the Mount Evolution REQUIRED to use the Eidolon as a mount? I would think yes since it exists but since the Eidolon starts out as a very intelligent creature and it does not use tricks or training I cannot see any rules reason it cannot.

What am I missing?

If the eidolon does not have the Mount evolution or if the rider doesn't benefit from the evolution (as for a rider of the same size), the eidolon is unsuitable for riding and imposes a -5 penalty to the Ride skill (CRB page 103). A GM might also decide that the eidolon can only carry a custom-made exotic saddle.