Royal Panaques
Aggressive to similar species otherwise very docile.
They are shy to begin with and will try to hide, eventually they will come out even during the day.
Temperment, grumpy old bugger. Chases my Oscar around the tank. Hasn't been able to corner him and do any serious damage, and the Oscar is getting bigger now.
But don't mistake big plecos for peaceful wimps, they are the armoured bulldozer of the aquarium world. If they get annoyed they can open a can of woop ass on most other fish.