If you see one, and are trying to fight or knock it out, i recommend going to a ledge above the rex, which in where it spawns can happen, make sure to use a good level longneck or other powerful tool if you want to kill one, and relocate if it gets close (they are not too fast). To knock one i recommend tranq arrows or darts (arrows if you have a good crossbow, they cheaper) and if you knock it just feed it prime meat or mutton, wait and then you get a Rex. If you have a tame that can outrun it you could relocate much easier

Easy process to find a tek rex on the island in UME:

1. Go to carnivore island

2. Bring a strong tame that can decimate every creature there (i usually bring a carcharo cuz it can 1 shot almost every creature there)

3. Kill any raptor, sabertooth, carno, argy, and rex that spawns there and continue for as long as you can until you see a shiny, metallic rex that looks robotic

4. If you're on singleplayer, then here are some bonus tips:

4a. I recommend backing up a save file every time you see a supply crate reach the ground since every time you enter your game a supply crate will be guaranteed to spawn near you shortly after. (You can also use this to farm keys since you wont need fo fly miles at a time to check every supply crate since if you're at carno island every supply crate that spawns WILL ALWAYS be on carno island unless you leave) If you can't kill everything on carno island once before the first supply crate reaches the ground, then back up a save every time you kill a rex or 2.

4b. If you find an alpha raptor, carno or rex and you need better gear you can back up your save and then kill the alpha to have a chance to get hide armor, sickles, pikes, metal axes, metal picks, crossbows, bows, and finally simple pistols (sorry this doesnt include swords). Chitin armor can also drop from alpha carnos and alpha rexes, and when paired with a high enough difficulty level and good enough luck, you can get a mastercraft or ascendant piece of chitin armor that can be stronger than a primitive flak piece (100 armor)

If your looking for a good level rec trust me it all DEPENDS ON YOUR LUCK I've been looking for a max level rex for alpha boss fights and I keep finding shitty rexes the highest I encountered were 125 and lvl 100 on beaches . I also found a 145 when I had no tranqs and when I came back it despawned . Just remember if you need an boss fighting army you need a rex army with good stats noy just levels

One day, back in 2016, I was living in a thatch hut on the black sands on the island. I heard a noise outside, and I saw a Rex fighting a Bronto. The Bronto died, the Rex victorious. Then it turned to me. It lumbered towards me while I was trying to close the door. The Rex started biting at my hut for around ten seconds, while I was crouched in a corner, waiting for my fate to be sealed. Then, as soon as it started, the Rex saw something else and went after it. I left the encounter unscathed, and my hut was also suffered minor damage.