posteulo

Where the extra e comes from. Post plus -ul- plus -o ought to give postulo, and it would, except that postul- is already a root of its own: postuli is to demand and postulo is a demand. So the word takes a linking -e- and comes out posteulo. PMEG says as much in its section on prefixed elements, listing postulo -> posteulo as an e-ending added purely to make the word comprehensible.

That is worth learning as a pattern rather than a one-off. A one-syllable particle put in front of a suffix can pick up an -e- when the bare join would collide with something else or become unpronounceable, and this is the commonest example of it in the language.

Two senses, one word. A posteulo is whoever comes after: your descendants down the generations, or the person who takes over your job. The opposite in both senses is antaŭulo, which needs no linking vowel because antaŭulo collides with nothing.