I've asked two different therapists for advice on how to deal with Koopa's fear of toilets, and they both recommended two things: 1) talking to Koopa about the toilet monster that exists in his head, drawing it and so on, to help him externalize his fear; 2) gradual exposure (sort of like what we did that time in the library) to gradually get him used to seeing different toilets so that he can eventually conclude himself, from the experience, that nothing bad happens when he looks at them (and eventually uses them).
Koopa has sort of already been doing #1 for a long time, externalizing his fear. He often plays "toilet", pretending that a closet is a toilet stall, for instance. One of his favorite toys is a little wooden toilet from a miniature furniture set. He likes to tell stories about a toilet, and asks me to tell him stories about it. He often asks us to show him images of toilets on the internet. He even wanted to be a toilet for Halloween (but it didn't work out). So he's already been doing a lot of work on his own, so what he needs now is a little more active involvement form us.
One evening I proposed him to draw a toothed toilet, but he wasn't up to it that time. Then I had another idea. I suggested that we draw teeth on our toilet at home, so that I also know what a toothed toilet look like. I took a washable marker and drew teeth following Koopa's exact instructions. The teeth had to be pink, and the tongue had to be green. Like this:
We played with it a little bit, and "fed" it toilet paper. Koopa seemed to like the game, and he wasn't scared, but then he's never scared of our toilet at home. This was my first introduction to the monster, and I hope to learn more things about it in the next little while.
The most useful piece of advice that I got is not to try to fix things, and not to try to fix him, but to try to understand him, see the world through his eyes, and eventually help him find his own strategies to deal with things. I'm still working towards this mindset.
Also, its important not to try to convince him that the toilet is nice, because for him it is not. This is a mistake I made many times in the past. As a friend of mine who is a counselor said, "if you try to disguise the toilet, the monster will always be there". I'm thinking about my fear of caves/airplane bathrooms or other small spaces: if somebody tells me that an airplane bathroom is my friend, that will just make me feel angry and lonely in my fear. It doesn't freaking matter how nice it seems to you, or how objectively safe it is. I'm freaking scared and that's what matters. This is my reality. It's the same with Koopa's fear of toilets, I think.
So, I'm trying to see this situation as an exercise in empathy. An opportunity to get into Koopa's shoes and see the invisible to the eye that he sees. Better start learning this skill now than when he's fourteen and his world is full of totally different monsters.