It was made by a famous BMX manufacturer, but this is most definitely a mountain bike. ATB stands for All-Terrain Bicycle. The brand name is a double-edged sword: respected by collectors, but the public now thinks of Mongoose as a tarnished Walmart holding.
You're right about it being a 1985. The previous owner changed out the rear derailleur and thumb shifters for indexed 6-speed Shimano Deore. The seat has been replaced with a spongy gel saddle. The right side pedal cage has been partially bashed off. The frame has been stripped of its decals. The tires are newer and don't appear to be cracked.
What's interesting to me is that some of the things that elevated the ATB Pro above the ATB aren't as desirable now. The roller cam brakes were an evolutionary dead end with higher mounting points than cantilevers and the unicrown fork became so ubiquitous they don't get a second look.
When you sell it, I'd raise the seat a few inches like the catalog photo, remove the cable lock & holder, give it a quick bath with dish soapy water, then rinse. Find a plain background outdoors, then take new photos that'll really show off the chrome (it appears to be in good condition). If you find rusty spots on the chrome, you can polish it with wadded up aluminum foil that's been wetted with plain water. The water will get black as it works. Wipe clean with a wet rag.
The chrome is really the biggest selling point here. People like shiny bikes! If you get some good photos, I think you might be able to get $200 for it, a price other vintage mountain bikes struggle to reach. Adjust downwards if it's snowy and frigid in your part of the country (prices dip in non-riding season). Of course you can just hang on to it and list it in the Spring. Adjust down if you don't live near a major metro area or college.
Good luck!