Learn Croatian: Zmija je gmaz, odnosno ona nema noge. - A snake is a reptile, or rather, she has no legs.

While both can sometimes translate to "or" in English, odnosno is used to clarify, rephrase, or give a more precise explanation ("that is to say" or "or rather"). If you used ili here, it would sound like a choice between two mutually exclusive things: either the snake is a reptile, OR it has no legs.

Why is the snake referred to as ona (she) rather than an equivalent of "it"?

In Croatian, pronouns must match the grammatical gender of the noun they replace. Because zmija is a feminine noun, it takes the feminine pronoun ona, regardless of the biological sex of the animal or the English preference for using "it".

Is nema just the negative form of ima?

Yes. A quick refresher: the verb imati (to have) is one of the few verbs in Croatian where the negative particle ne merges directly with the verb to create a single word (nemati). Therefore, instead of saying ne ima, you must say nema.

What case is noge in this sentence, and why?

It is the accusative plural of the feminine noun noga (leg), acting as the direct object of the verb nema. Although older or more formal Croatian sometimes uses the genitive case for objects of negated verbs (known as the Slavic genitive), modern everyday Croatian almost always sticks with the standard accusative.