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.
The term  axiom may also be used to refer to a background assumption. Example of a postulate: Through any two points in a plane there is exactly one straight line. A theorem is a statement that can be proven to be true based upon postulates and previously proven theorems.