The majority of units that drop in the map are pretty useless, and you won't find them good for anything other than leveling up your Rare/Honour Summoned units or selling them for Zel.
Maps in St. Lamia do have a chance of dropping some pretty powerful units that can be evolved into 4 star forms (and later 5 forms when the update comes).
The encounter rate for these units is very low and the drop rate is even lower, but you will eventually capture the unit you're after if you keep trying.
Below is a list containing the unit name and the map they can be encountered/captured on.
