SELECT user, host FROM mysql.user; yields output like the following for us:
+------------------+---------------------------------------+
| user | host |
+------------------+---------------------------------------+
| % | % |
| plutot_admin | % |
| plutot_dev | % |
| plutot_intrn01 | % |
| plutot_intrn02 | % |
| plutot_ntrn01 | % |
| plutot_ntrn02 | % |
| plutot_tech | % |
| plutot_ntrn02 | * |
| plutouser | * |
| ... | ... |
% is wildcard host, but what is the significance of * in the query output?
MariaDB (but also MySQL) isn't restrictive and doesn't check the validity of a hostname (RFC 2181) and therefore doesn't claim if a host name contains an asterisk or another invalid character.
The asterisk will not be interpreted as a wild card character, wild cards are limited to percentage (%) and underscore (_).
That means someone created the user with CREATE USER 'plutouser'@'*' command, likely with the assumption that the asterisk will be interpreted as a wildcard character.