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.
What is axiom - Definition and Meaning. Axiom : A statement which is true but does not have a proof is known as axiom or postulate. Example : Let a and b are real numbers. In addition and multiplication, the results are also real numbers. eg., a=2, b= 4, then a + b = 6, a * b = 8. Here 6 and 8 are real numbers.