In my opinion, the best bet is to take it out to an area of wilderness and let it go. I'm sure thats not what you wanted to hear. But putting it in with a garter snake is ill advised as whether it would or wouldn't eat it, there would be a heavy stress factor being subjected like that over a period of time. ALso if you should read down the posts on this sight the horned lizards main diet is ants. Usually big red ants. Because the care and husbandry for these animals is not for novice's and even people who have had many different species of lizards sucessfully have had major problems in keeping these lizards. There is a place where you can buy ants but your talking $10.00 a pop for 500 and it's not uncomon for them to eat 50 to 200 in a setting depending on the animal. So that would be a big committment. Then you would also need a uvb/uva bulb which given the different types or manufacturerers will run you around 30 to 50 dollars, then you'll need a basking bulb which is usually around 12 dollars and a light fixture for the uva/uvb bulb whioch is about 12 to 20 dollars at home depot. You also need a bulb holder for the basking bulb which is about 8 to 12 dollars depending on the one you purchase. You didn't say where you live or what specie, but at a minimum you should have a 20 long glass reptile cage and thats about 50 dollars, plus the reptile sand for it. YOu'll need to set up the cage so the abmient tempreture is somewhere around 85 degrees on the cold side and up to 100 or 110 depending pon the specie. So this is just the "get started scenerio" which unless I interpreted you question incorrect, you seem to not be at the point where this would be a healthy project to induce to this little horned lizard. I would suggest that you do some reading up on them and there care read this site as well and maybe first try some other lizard and keep them for a period of time first. Tackling a horned lizard in captivity is not an easy job. It takes a big expense as far as feeding and investment for the correct cage arrangement. There are many people that try and feed their horned lizards a regimum of non ant food such as crickets, mealworms, and wax worms and superworms, and horned lizards are opportunisic and will eat other insects and probably do, but scat anylsis of horned lizard poop is 90 percent ot more ants, therefore most of those that have been sucessfeul at keeping horned lizards must have a committment to providing ants. THis is a generality here and not specific because I don't have enough information from you, but I feel that that this little guy would do best if you let him go, and it was very admirable that you have rescued him. I hope you won't be offended at my answer, others may have more to add--but unless you are experienced in raising lizards specifically, I would opt for setting him free. Thanks and let me know................................