It is very rare.
Maybe a few days after they were released. My crew met another on shipwreck who were new to the game and decided to 'Trade' cargo to gather the stolen commendations.

As we were gathering all of our loot and playing with out pets...the Skelly spawned up 5 feet from us. Funny...it didnt even make it out of the ground before we all killed it in a panic.

@filius-aurorae My crew and I have only found one so far and it was actually during an arena match on a fort island. It paid out 140 some.

@filius-aurorae Saw one in the tunnels of Sailors Bounty while doing art of the trickster. Still have never seen the shrouded ghost....

@filius-aurorae I've found three now but I consider myself to be really lucky. All three were while I was solo and presumably on a dead or near dead server. Two were on large islands. Shipwreck Bay for the first and Cannon Cove for the third. The second I encountered deep inside the shores of gold while near the end of Tall Tale 9. They spawned nearby as if they were normal skeletons trying to attack me. The music that plays and the sound of coins make it very obvious that they are different than any other skeleton though.

@MasterBetty89 I have been curious how much on average they will award you. I recieved 146 from my first and 125 from my second so that seems to be in line with what you earned also. I'm still kicking myself for missing that third one though. It would have gotten me even closer to a new monkey companion to join me on my adventures.

@filius-aurorae

Apparently they are pretty shy but I found one on Fetchers Rest last night!
It paid out 254 coins.

@filius-aurorae hi there, for me I just played the game treated the ancient skellies the same as the shrouded ghost. I have encountered one skellie in my travels this far, it payed out 148 ancient coins.