They have different symbols and colours. There’s a purple one, a red one with weapons, a green cross, this one has green slashes. But I have no idea what any of them mean. 
This must be something new, because it wasn’t there when I was playing a few years ago.
Those symbols depend on the enemy. The pic symbol is for the enemy who is Swift, basically fast. When targeting an enemy their name (e.g. Malicious - Regenerator) aligns with these symbols.
Arcane enemies have a crossed O
Regenerators have +
etc.
The symbols basically allow you to spot a crucial enemy within the mass and divert your attention where it is needed.
# for Swift/+speed
superman S for Unstoppable/+CC res (500%/immunity?)
red crossing swords for Vampiric/lifesteal
shield for Defender/+DA(/armour?) field command buff type deal
hourglass for Timewarped/+slow debuff (and sometimes a heal too)
Thanks people!