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.
Axiom - A statement about real numbers that is accepted as true without proof. Deductive Reasoning - A form of reasoning by which each conclusion follows from the previous one; an argument is built by conclusions that progress towards a final statement. Inductive Reasoning - A form of reasoning in which a conclusion is reached based on a pattern present in numerous observations.