So I would like some help from the collective knowledge here, and in turn I will share what I have found. Please keep in mind that this is not perfect fact, but I have found it to be pretty accurate.
Midnight Panther - seems like the spawn interval is a minimum of 3 days. Always spawning by the 7th day after it last been seen. There are 3 positions that it can spawn on. I've always seen it spawn in the Far East spawn first. Several hours later (5-8) it will spawn in the south spawn, and sever hours after that (5-8 or 10-16 hours after its first seen) it will spawn in the north. Once I found this out I've been able to tame it on several characters and killed it many times. I would say in the last 6 months I've run into the panther at least 10 times across a couple servers. Watch in chat for someone to Brag about killing it to get the initial spawn, or ask in chat if anyone has seen it lately.
Undead Cavebear - I feel (again not fact) that the cavebears spawn is way more predictable than the panther. 1 bear will spawn down the first pathway to the east, 5 (or so) hours later 1 - 2 will spawn down the near western path, and 5 (or so) hours after that 1 - 3 spawn in the main hallway to the west. This cycle repeats itself every 3-4 days like clockwork. Once you catch wind that a Cavebear was found, you can keep track of it with relative ease. Since I put this info together, ive run into 20+ cavebears over the course of a couple months across a couple servers.
So now I need help. I can't find the giant lobster to save my life (Crustacea Gugantica). I'm currently sitting in treasure island spawn with a stealth ring. I know for a fact that one spawned on my server yesterday around 5pm (I missed it by minutes judging by the dead body). My question is, does anyone know roughly how often he spawns in the treasure island location? Thank you!!