Let's say I ask someone to make a 3d model of a monster, can I then publish his work on a modding website? Is there any other thing I need to be aware of? Do I have to put his name to credit him for the model, or not? I am just wondering, because I am thinking of getting the help of some freelancers.
Unless your contract specifically states that you own all rights or the right to resell/redistribute the work... then NO you do not have the right to resell or redistribute anything under any circumstance.
If you do not have rights, then adding a "credit" is meaningless. However, a contract can stipulate that reselling or distributing is permissible if a credit/attribution is included. Creative Commons licensing often has this stipulation.
In short.... It depends upon what the contract agreement for the work states. Without a formal contract or by default, the artist owns all work he/she creates and you have no rights to resell/distribute anything (at least under United States laws - laws may be different in other locations).