This is a resource I have used for a while now and he does a fantastic job of researching the monsters and converting them to PF1E for play. I will tweak them on occasion if I need to but it is a great reference site and a great starting point on conversions.
Was formally called:
The Chronicles go back to 2012.
The Codex go back to 2016.
Tons of great conversions and new monsters as he is avid reader of monster mythology, folklore and such.
If you do a search for the [monster name] AND Creature Codex AND Tumblr or [monster name] and Creature Chronicles AND Tumblr you can go to the page directly you are looking for.
EDIT - let me correct myself-. Codex is owned and maintained by dmiurge1138 and Chronicles (which is no longer updated was owned and maintained by filbypott. Both are nice resources for old 3.5/3.0 and even the older ones that never got ported over to the newer versions plus some cool creations of their own.
A friend of mine linked me to this thread. Thanks for the kind words! The Codex and Chronicle are indeed separate projects, though Demiurge is a friend of mine and has my full blessing.
Incidentally, both blogs are named after the EN World Creature Catalog, which similarly converted AD&D 1st and 2nd Edition monsters to 3.5.