i've found that almost all animals that can kill your character are bugged.
when u set a velocity or gravity, the settings automatic go to deafult.
or even worse, you set a velocity of 50 and gravity 0 for example, and the animal starts to fly.
i strongly believe that this is one of the biggest bugs on the editor.
The velocity/gravity settings only revert when you try and edit the settings again. As long as you don't mess around with an animal's values, they will not change by themselves.
As for the second "bug", I believe you didn't set up the animal's properties correctly. First gravity/velocity value affects left/right movement, the second one affects up/down movement. For instance, if you wanted a maggot to start crawling to the left while initially being on the ground, you'd set the first velocity value of the animal to something like -30, and you'd set the second gravity value to -500 or so (this will make the maggot fall to the ground, where he'll start crawling).