Now, the obvious answer is, of course you can eat meat and fish cold but is there something you're not telling us?
Why ever would you think that cold meat or fish would, "become poison"?
Added, I think you must be listening to some odd people! Haven't you seen pre-cooked (and cold) meat pies in shops? Have you really never eaten cold meat? In the UK just about every delicatessen counter has cold meats, including beef, for sale, all kept and served at room temperature.
I'd really love to know how cooked food could possibly become poisonous as it got cold!