"A chicken is not a mammal."
Translation:Koko ne estas mamulo.
'couse "ne" should be before the verb. It's not English ("is not"), but Esperanto. Watch the other languages: Spanish: Yo no hablo (I am not talking), Polish: Ja nie mówię, Slovak: Jo nie hovorím etc.
It's never a good idea to apply the rules of language A to language B. In Esperanto, we're free to have [noun adjective] or [adjective noun] and we're free to have [verb adverb] or [adverb verb] and "ne" is an adverb like any other.
Now, if there's an actual rule that makes negation special and thus must always come before the verb, that's different. But if all we have are general rules to go by, then it's reasonable to think that "estas ne" is just as correct as "ne estas".