The husk and stray are counted as sub-variants of existing mobs as opposed to entirely new mobs, hence why you are unable to simply type /summon Stray or /summon Husk. There was supposed to be spawn eggs for them, but it seems they were removed either intentionally or by accident in the final release.
Axiomatic method, in logic, a procedure by which an entire system (e.g., a science) is generated in accordance with specified rules by logical deduction from certain basic propositions (axioms or postulates), which in turn are constructed from a few terms taken as primitive.
An axiom or postulate is a statement that is taken to be true, to serve as a premise or starting point for further reasoning and arguments. The term has subtle differences in definition when used in the context of different fields of study. As defined in classic philosophy, an axiom is a statement that is so evident or well-established, that it is accepted without controversy or question.