Where the order comes from
Thorium, Spirit, The Stars Above and Calamity Fables each publish a boss progression chart on
their own wiki that already interleaves their bosses with the vanilla ones. Those charts are
merged here on the vanilla bosses they share — so “after Skeletron” is that
wiki's own placement, not a guess. Vanilla's order is its own Bosses page.
Where it does not exist
Fargo's Souls publishes no chart — but its boss pages say in words where each fight
belongs (“intended to be fought before the mechanical bosses”), so those
are placed from their own sentences, quoted on the row. A placement read that way is held to
the tier its wiki files the boss under, so a post-Moon Lord superboss cannot drift into
pre-Hardmode. Mini-bosses are the only ones left unordered: no wiki sequences them, and
guessing would be worse than saying so.
Reading a row
The colour down the left is the mod that adds the boss. Summoned with is the item you
need to start the fight, quoted from that wiki. Gear up jumps to the Loadouts checkpoint
you want to be equipped for. Tick a boss and it greys out; the next unbeaten one is called out
at the top.