Remember that tegus, even those that have been farm bred for several generations, are wild animals. They still have all the instincts and behaviors that serve them in the wild. These same instincts and behaviors same time annoy and confuse us, particularly those who think of lizards like domestic animals.
(I'm not saying you do, just being complete in my responce.)
I keep mine tegu out doors from Memorial Day til Columbus Day. He has an enclosure with hiding places and areas for him to dig burrows. These help him feel secure. Wide open spaces are scary for wild animals particularly if they are small. My tegu is now just over four feet and about 20 lbs. I bet he still gets scittish outside this summer.
In the summer he dives for his borrow when anyone walks by. He'll stay out for me if I move slowly. He gets a bit wild inthe summer, but usually calms down when I grab him. He's much calmer indoors for the winter.
The tail rattling behavior is VERY intersting! Many harmless New World snakes (Colubrids) will do that when frightened, but I've never heard of a lizard doing it. Very Cool!
Thank you,
Brian
Here's an old picture of my tegu enclosure.

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Brian
"The Earth was not given to you by your parents. It is loaned to you by your children."
1.0 Rhinoceros Iguana
1.0 Argentine Tegu
1.1 Bearded Dragons
1.0 Australian Water Dragon
1.0 Jeweled Lacerta
0.0.2 Blue-tongue Skinks
1.1 California Kingsnakes
1.0 Children's Python
0.0.1 Red-earded Slider
0.0.1 Giant Mexican Musk Turtle
3.4 Russian Tortoises