New Friendly Mob: the Experience Bottler

This would be a mob you can trade with, perhaps they're trapped in Ancient Cities like Allays are by pillagers, or perhaps they're found wherever the Ancient Ruin portal leads. Main thing is that it takes XP off your bar, and gives you experience bottles instead. This would fit with the theme of sculk, since that spreads by eating experience.

I feel like the Experience Bottler should be a mid-way between a Villager and a Piglin, where it's not necessarily hostile to you, but it is distrustful. Freeing it and giving it food (maybe bread, cookies, or cake, since those foods fit with being "intelligent") will make it willing to work with you. Maybe it gives basic quests, like fixing or turning on some of the redstone machines in the Ancient City, or gathering cookies for it.

I'm imagining it as a kind of blue snufkin, that you save from the Ancient City. When you make it trust you, it's willing to help you get the Ruined Portal in the Ancient City working again, and it disappears through it. Before going through the portal, it gives you an item that lets you summon it, and when summoned with this item in the overworld it bottles your experience for you, in exchange for cookies or fish.

For looks I imagine it should either be a small allay-like creature, or larger and looking Warden-like - perhaps both. And like snufkin, it also likes to fish :)