It's just a normal system user. I have no idea whether those need to be attached or how to do it. Can someone explain?

@Anomie Would you know what's expected behavior here?

Special:CentralAuth isn't showing it because there's no entry in the CA localuser table. I don't know why there isn't such an entry; I see the specieswiki user was created 2016-03-19, so it's possibly some bug that has since been fixed.

I misstated things earlier: it's not showing up at all because there is no entry in the localnames table, not localuser

The fix for the specific fact of these missing table entries would likely be something like this, using mwrepl:

$centraluser = CentralAuthUser::getMasterInstance( 'FuzzyBot' );
$centraluser->addLocalName( 'specieswiki' );
$centraluser->attach( 'specieswiki', 'login' );

If you don't have access to do that yourself, let me know.

EDIT: Better merge method.

In T141465#2564284, @Anomie wrote:

$centraluser = CentralAuthUser::getMasterInstance( 'FuzzyBot' );
$centraluser->addLocalName( 'specieswiki' );
$centraluser->attach( 'specieswiki', 'login' );

Shouldn't that be getMasterInstanceByName?

Yes, it should.

https://species.wikimedia.org/wiki/Special:CentralAuth/FuzzyBot now shows it as attached, though does not specify the method. Should I do the same for the two wikimania wikis for which it displays unattached?

You could.

It is showing the method, it's just that the icon for 'login' is a completely transparent image.