Quote Originally Posted by No brains View Post
Q85
a What is the creature with the lowest CR that can cast Detect Magic at will?
b What is the creature with the lowest CR that can cast Detect Magic at all?
c Can creatures use ritual casting if they have spellcasting from a class that grants that feature?
A85a: According to DDB it's Warlock of the Great Old One (VTtM) that's a CR6 creature.

A85c: No. The class feature that allows ritual casting specifically states spells of that class.

Example from Cleric (underline emphasis mine):
Ritual Casting
You can cast a cleric spell as a ritual if that spell has the ritual tag and you have the spell prepared.