[Pako]

Not sure if you'd consider this a "complete application" that is bigger than a "Hello-World-esque example" but I've worked on a Discord bot in SWI Prolog[1] for a while with a friend. It was an interesting experience and although a lot of the work (e.g. SWI has a ready-made library for interfacing with WebSockets and sending out HTTPS requests).

Some interesting applications of Prolog specifically include using predicates to filter messages by certain criteria (e.g. if it was sent by the bot's account or not), being able to hot-reload by invoking the make/0 predicate, and homoiconicity to (in theory) easily evaluate random code supplied by a user.

[1]: https://github.com/prolord-pl/prolord