Rear WHeel has wiggle in it.
 

I have an older MNT bike, ive noticed the rear wheel has some wiggle room side to side. The front wheel does not do that. Is this normal? my first thoughts were wheel bearings, but i dont know much about bike mechanics. it rides fine, just wondering if its something i need to start watching out for. thanks.

Is it the rim, with the "wiggle" suggesting that the wheel needs truing or the hub, suggesting that the bearings need adjustment?

Or the tire, suggesting that it is not mounted straight?

If you can shake it side to side the bearing does indeed need adjustment. If it's solid when you try to move it side to side but wavers at the brakes when turning it's spokes/rim. If both of those are OK but the tread on the tire moves back and forth the tire needs attention. In any case, don't watch out for it, have it fixed!

Older may be a freewheel hub and a broken axle.

loose cone nuts

This covers the bases. except that bearing adjustment should be attended to ASAP, whereas minor mis-alignment can be ignored or put off as long as there's no rub on the brakes.

Always a possibility, but will be discovered when the bearings (cones) are adjusted.

Putting aside the likelihood that it is the bearing, with a less knowledgable rider I prefer to be on the safe side and get it attended to no matter what. It's a good habit to look at problems sooner rather than later. Even if it's "only" the tire it could be a poorly seated bead about to blow out, or ruptured cords.

Is it a wiggle or a jiggle? Or a wobble or a shake or a shimmy?