According to export_descr_ancillaries.txt, there are 4 retinue who get command:

1. Shipwright (1) linked to shipwright building, only available to newly created admirals. 4% chance for every new admiral.
2. Seamaster (2) linked to dockyard building, only available to newly created admirals. 20% chance for every new admiral.
3. Navigator (2) linked to movement. 5% chance for every admiral that has 0 movement points left.
4. Priest of Neptune (2) linked to neptune temple (second level). 15% at creation.

There's also one unique ancillary linked to the academy (that goes to generals, not admirals) and is turn limited. But i'm not convinced all of these unique ancillaries actually exist, since the probabilities indicated in the file are too high (80% chance for samos)