Learn Finnish: Tämä on maailman viidenneksi harvinaisin nisäkäs.

In Finnish, to say 'the [number]th most [adjective]', you put the ordinal number in the translative case (ending in -ksi). The superlative adjective that follows it, harvinaisin, takes whatever case the sentence requires. Since this sentence is a basic 'A is B' statement, harvinaisin stays in the nominative.

How do we get viidenneksi from the base number viisi?

First, you form the ordinal number for 'fifth', which is viides. To add the translative ending -ksi, you need the inflectional stem of viides, which is viidenne-. Attaching the ending gives us the required viidenneksi.

Why does this sentence use the genitive maailman instead of saying maailmassa (in the world)?

When expressing 'the [superlative] in the world', Finnish strongly prefers using the genitive form maailman (literally 'the world's') placed right before the adjective phrase, rather than using the inessive case (maailmassa) later in the sentence.

The word for mammal, nisäkäs, has an interesting ending. How is this word constructed?