Model stretches when posing in blender I have a problem with my model that I can't seem to fix. I looked all over the internet, but I can't find any answers. 

I'm making a model for a demo and when I pose it in pose mode, some of the vertices get left behind. how can I fix this?

Also I'm having this strange issue where the tail stretches when I pull on the arms. How can I fix this as well?
That means that a few vertices aren't weighted to any of your bones. Select your rig, go to Pose Mode, then select your mesh and go to Weight Paint Mode. Now select the bone that should be controlling those vertices and using the Add Brush add some weight to those vertices and they will automatically update and follow the bone you have selected. You can make two or more bones control a vertex by giving that vertex a weight from multiple bones.