[xpyder]

Natur EnemyCastBar - popular, out of date. Owner may be updating at some point

Enemy Ability Bar - not just cooldowns, shows bars with all the abilities of your enemies. "The addon is fully customizable...and add abilities by your own."

Aesa (Enemy Cooldown Tracker) - Popular, looks like mostly what you want but doesn't say how customizable it is.


p.s. I found all these on curse, I don't know how many of them are on WoWI. I searched "enemy" and got few results so you could try that too