 Euclid-class SCP objects kind of fall in a middle road of the SCP Foundation when it comes to object classes. Euclid-class SCP objects are a lot more unpredictable and require more containment resources to contain them. A great example of what a Euclid-class SCP object would be would be anomalous humans, humanoids, or creatures. Basically anything that can think or have a will of its own will usually automatically get a Euclid-class rating on it. If it's unpredictable and it's hard to understand it will most likely be a Euclid-class object until such time they can figure it out more easily.