Learn Finnish: Eläin on syöksymäisillään, kun se saalistaa. - The animal is about to dive when it hunts.

It uses the fifth infinitive, which shows an action is on the verge of happening. You take the strong vowel stem of the verb, which is syöksy-, add the fifth infinitive marker -mäisillä- (using front vowels to match the word's vowel harmony), and then attach a possessive suffix at the end.

Why does the fifth infinitive need the -än ending here?

The fifth infinitive must always take a possessive suffix that matches the subject of the sentence. The subject here is eläin (third person singular), so we use the third-person possessive suffix. The third-person suffix takes the form of lengthening the final vowel and adding -n (-Vn). Since the preceding vowel in the marker is ä, we add -än, making it -mäisillään.

Could I use this same fifth infinitive structure to say 'The animal is about to migrate next month'?

No, the fifth infinitive is strictly used for actions that are on the absolute brink of happening right at that very moment, often physical actions like jumping, falling, or crying. For an action planned in the future, you would use a different structure, such as the verb aikoa (to intend).

The verb saalistaa means 'hunts'. Does it come from a noun?