Is there a situation where MySQL creates a nonclustered index automatically?
The PRIMARY KEY is a UNIQUE key; it is also "clustered". (Note: Clustered)
In the absence of an explicit PK, the 'first' UNIQUE key containing only non-NULL column(s) is effectively promoted to be the PK.
If that fails, a hidden 6-byte number is used for the PK.
A UNIQUE key is an INDEX, plus a uniqueness constraint. (Note: may be clustered)
An INDEX is a BTree that is used for efficient access.
A FOREIGN key creates an INDEX if there is not already a suitable index. (Note: Non-clustered)


The 3 "Notes" are implicitly-created indexes, but not necessarily non-clustered.