A guy did this, so I did this. I will do some more in a bit.
This is a crazy, ugly-looking, fucked-up monkey. It's only notable trait is that is seeks always to demonstrate its dominance over all other nearby animals. That includes all members of the Party.
It does this by climbing up their body and sitting on their head and staying there until they get rid of it or until they are no longer the most dominant person around.
The monkey is dexterous, violent, nimble, clever and persistent. One HD, but a high AC due to it's smallness, maneuverability and the fact that when you're trying to kill it, it's probably on someones head.
Having a Lord Monkey on your head does clearly prove to everyone around that you are the dominant person in the vicinity. If a guy walks into a bar with a monkey on his head you know he's not to be messed with.
However it does mean you have a fucking monkey on your head so the charisma benefits and minuses tend to even out.
If the PC's walk into a room with a very high status person, the monkey will do a quick estimate and change heads if necessary. It always knows who is the most powerful or highest level person in the room. Always.
No. Always. Even then.
A good way to get rid of Lord Monkeys is to spend a lot of time with Dragons or Kings. Of course if this individual does end up with a monkey on their head, they will not be happy.
Everyone the PC's meet will believe absolutely that harming a Lord Monkey in any way is incredibly dangerous and VERY bad luck. They will tell the PC's this if asked and volunteer the information if not asked. They are wrong.