Aberrant eidolons can select other unchained evolutions as normal, provided those evolutions don't list alignment subtype or a particular type of eidolon (like claws and many other natural attack options do). The aberrant base form also removes any base form-dependent options. That said, tentacle mass is a good deal for natural attacks. (I forgot to check if that was primary or secondary...) It works much better than the fey or shadow eidolons, since it gets its own additional evolution options.
I should be more precise. I'm reacting to the fact that for any other natural attack evolution, the aberrant eidolon is unlisted and therefore, de facto, the only attack mode legal for the aberrant eidolon is the tentacle mass.
I doubt this was intended. Even the model Aberrant base form has a bite attack listed, yet apparently this is not legal.
Clarifications on legal natural attack evolutions for the aberrant eidolon would be useful as I think they were inadvertently omitted.