Negating both the primary verb and a relative clause

"a" is ambiguous between "any" and "some". If it is within the scope of negation, it means "any". Put "any" for the "a" in "a lizard" if you want the sense to be that the preceding "not" includes the "is a lizard" predicate.

In Constraints on Variables in Syntax, John Ross points out that the influence of a negation extends to the interpretation of "any" downwards in the structure through an unbounded number of indefinite relative clause constructions: "Hank would not accept any offer from any waiter in any restaurant in any neighborhood of Chicago of any hotdog with any ketchup on it, for fear of reprisals." (Ross's example was better than mine.)