Is there a hidden ending in this game?

From what I can tell, no. Basically if you don't know about this already, many Capcom arcade games in the early to mid '90s had some kind of extra bonus ending, usually an extra credits sequence, that you could view if you met certain conditions whilst playing the game. For example, if you beat The Punisher without using a continue, you'd get a different credits sequence.

However Cadillacs and Dinosaurs doesn't seem to follow this trend. I've tried beating the game on one credit, beating the game without losing any lives, trying to get a really high score, but nothing seems to work. The only other option I could think of was maybe defeating all the evil (orange) dinosaurs you encounter throughout the game. The fact that there are unused developer mug shot sprites in the game still makes me think there is some kind of hidden ending in this game that nobody has discovered yet.

Wasn't this game ported to the SEGA CD and PC?

No, the game you are thinking of for those formats was Cadillacs and Dinosaurs: The Second Cataclysm (pictured above), which was released in 1994 and developed by Rocket Science Games. This title was a completely different game to the Capcom arcade beat 'em up, and it had you playing as Jack and Hannah as they raced through different landscapes, shooting at dinosaurs and the like.

Wasn't this game based off a cartoon?

No, the game came first. Whilst they were both released in the same year (1993), the arcade game was around from April whilst the cartoon didn't start to air until September. The cartoon keeps the same basic plot, with the dinosaurs returning and wrecking havoc whilst Jack, Hannah and others try to keep things under control. However, the likes of Dr. Fessenden and a council of evil governors led by Wilhelmina Scharnhorst are always up to no good. For a lot more info on this, check out the overviews of each episode Ragey did here.

Is there any guaranteed way to get the M-16A1?

I don't think so. It seems to be completely random to whenever it appears out of a breakable object. For example, in the second part of level 1, sometimes it will be in one of the statues, but a lot of the times it won't.

Is it possible to beat the third level without using the Cadillac?

Yes, but be prepared for a very long level. The only reason you'd want to do this is if you are playing for points, because there are tons of enemies to kill here. Time is another major factor -you'll have to defeat enemies very quickly to beat the level without running out of time.

Can you defeat Hogg without losing the Cadillac?

Yes, but it takes a lot of skill. As long as you can avoid his grenades and keep driving into him, it is possible.

Why won't this game ever be re-released?

Most likely because of the licensing issues with General Motors. It could just be that General Motors (who make Cadillac cars) wanted to charge a lot of money for Capcom to release this on home consoles, and Capcom didn't want to pay it. I don't know for sure but hopefully, one day, this game will find itself as a downloadable title on current generation systems.

Was this game done on the CPS-1 or CPS-2 hardware?

The game is running on a variant of the CPS-1 board known as the CPS Q Sound. This hardware is mostly the same as the CPS-1 board, but had improvements in the sound generating department, hence why the game sounds so great. The reason the graphics are so good is just because Capcom were really making it look that spectacular.