I would say a mouse or gerbil or something small like that. Maybe a toad of some sort. A tarantula or an emperor scorpion can also live in a 10g. I wouldnt go with a rat though because they are tall enough to reach the top and they are very smart and will get out. I had a rat in a 10g i had it taped shut and had bricks on top of it and she still got out and was lost in my house for 3 days until my mom found her under the bathroom sink lol.