I'm inspired by the comments discussion on a (now deleted) question by XandarTheZenon. We're preconditioned to think in terms of a 2-sex lifeform, which is natural with diploid genetic material, and sexes that specialize with different reproductive strategies. This often leads to species that have sexual dimorphism. (At the very least, the sex organs are dimorphic.)
But that's not alien enough.
Why else would an animal-like species have different body plans within one species? In particular, it would not have the male/female division that Earth macrofauna life has. Reproduction could have a different number of sexes, or at least doesn't have the same kind of division where one partner invests most of the energy and sustenance and the other is non-essential to development, with different body plans and behavior evolving from that original source of difference.
How can an alien species have more than one body plan (and behavioral traits to match) yet individuals of all types are needed to make a family?
A human biologist would not find it immediately obvious that these are the same species at all. But there's a sound reason why there are different forms and furthermore they are all necessary as part of a family unit, rather than just being different lifestyles.