If you want a sorta big one it doesn't grow that much- a red eared slider
Well... Actually you can put a Red Eared Slider in your pond cause they like going outside so just make sure there is a shaded spot and make sure they can't get out and make sure they have food...Oh and if it is a baby Slider (I have one) you might want to put a fence or something over the pond because a lot of birds and cats and stuff tend to like to eat Red Eared Sliders... And Red Eared Sliders grow to be about a foot long or as big as a regular sized floor tile... So consider what I just said and if you don't have atleast 1 of the things above then don't get one...BE PREPARED... I just got one yesterday