Minecraft wiki entry for Development_versions, associated text: Found in random places around the world.
Provides a random variety of biome-specific item trades.
Trading times are limited, locked trades are not unlockable by conducting other trades.
Has 6 trades, all unlocked up front, once used up cannot be reopened.
Are always surrounded by a few trader llamas.
Are separate entities from villagers.
They appear to be villagers, but their spawn location, sounds, and trades differ.
Only one exists per world and despawns after 48000 ticks (40 minutes).