So far all I have about Sents is that they are born to people that are evil or bad in some way. The more points on their horns the more terrible the parent (these designs were random). They can't reproduce, they can be killed just like a normal human (thinking about having them never die from old age) but they aren't necessarily evil themselves, just deformed, if it's easier to think of it that way (however most end up bad due to poor upbringing).
They have the same hair, eyes, and body types as humans, but they have skull snouts, blue horns, black skin, animalish ears, and blue talons on lizard-like hands and feet.

 tiamat and  ouisch at the top right (nameless guy to the left)
Me, Farina, and  gatorfox

(feel free to make up your own. They exist in Genesis Saga, but I imagine there are a lot of them anyway)