ladubois

Yeah, I was kind of discounting crocodilians, already, for all of those reasons. ...However, now that I think about it, most of the larger lizard species (at least that I know of) are carnivorous, which probably raises another practical concern about trying to domesticate an animal that could - and may very well want to - fit you in its mouth...

Komodo dragons seem to only be about the size of a mid- to large-sized dog. Dogs have been used as pack animals, true, but they're pretty limited in that capacity. Speaking of dogs, they are perhaps the best-known example of taming a wild animal. There's also what appears to be an increasingly well-known Russian study that has successfully bred domesticated foxes. Iguanas are already domesticated and can get fairly large. Of course, it would probably be wiser to try using a lizard with a shorter and/or stockier tail, and without ridge spines. But they're related. Granted, so are birds, which are quite different from reptiles, but I'm willing to bet that ankylosaurus and triceratops are more closely related to modern lizards than modern birds. I will, however, admit my substantial ignorance even to what is known about the subject, so... Moving on. Elephants are fairly slow-moving animals, particularly compared to other pack animals. They're capable of bursts of speed, but I rather doubt speed has anything to do with their daily use. And as you briefly mentioned with musk oxen, there are all sorts of cattle that are used as pack animals or for labour, at least historically. The vast majority of these have pretty much already been thoroughly replaced by machinery, but that's more of a practical reason for why we haven't tried doing this, not a theoretical one for why we couldn't.

Probably a rather odd series of questions, but they really got me curious, and sadly, I have only the barest frame of reference for any of the fields that would be needed to start answering them. First off, would a camel- or mule-sized lizard potentially make for a better beast of burden in hot, dry climates? I really only have my intuition to go on, but it seems to me that reptiles are better suited to adapting to deserts and desert-like environments than other animals. They don't generate their own heat, so they seem less susceptible to overheating, and I imagine that cold-blooded animals need less food than, say, mammals of similar mass. I could be completely off on this, of course. Second, is there anything (aside from how incredibly long it would take) that would prevent us from theoretically breeding equine-sized lizards into existence? Ankylosaurus and Triceratops sizes show that it's been done before, so physical constraints like the square-cube law can clearly be overcome, but perhaps there's something environmental that keeps lizards so incredibly small...