Historically, units of measurement are often based on sizes of body parts. Probably the most obvious example would be a "foot" literally being based on the length of someone's (or everyone's, or anyone's) actual foot. Other examples include inch (length of a thumb, from tip to first joint), cubit (elbow to tip of middle finger), and span (from tip of thumb to tip of little finger, when spread as far as possible).
For most historical human populations that used these types of systems, this worked well enough for their day to day lives, despite the different sizes of body parts from one individual to the next. When that size variation was a problem, they could standardize, somewhat, by using the body part of a ruler/monarch/lord/etc. instead of just some random individual.
Enter the worldbuilding conundrum of populations that include both human and non-human races, some of which have distinctly differently sized body parts, or might even be missing a body part that another race has. The exact races present are not necessarily relevant to the question, but for illustrative purposes, I'll provide a couple quick examples. A lizard-human-hybrid type race would have a tail, that a human would not. A race of merfolk that are more fishlike than the standard trope might have webbing between their fingers or might not have fingers at all with fins or flippers in their place. Giants or dwarves/halfling-esque races might be mostly human shaped, but wildly out of proportion to humans in actual size.
In a world-building setting where at least 2, if not more, such races commonly and constantly interact, making body part lengths inappropriate as common units of measurement, what is the most likely basis for such a society to use as units of measurement instead?
First, apparently I wasn't clear enough that I was looking for answers that did not include the use of body parts of the races, at all. As in: body parts are off limits as an answer. The question was meant to ask for alternatives to body parts, not variations on the use of body parts or how body part usage could be standardized.
Next, a note about the monarch and other variations of standardization, I understand that a monarch creating a standard is often how things were done, but this question was meant to be about what that monarch himself might have based his official standard on, if not his own body parts or someone else's. Sure, his official bar of metal could still be used as the defined standard, but what did he base the length of his official standard bar of metal on?
Answers that basically state "they'd all just agree or learn to convert": sure, but what would they use as a basis before they learned to agree on one or convert, if not body parts?
Assume the location is actual Earth, with the only difference being the additional races present, and assume ancient and/or medieval, as anything more specific shouldn't really affect the answers, since it's about what people from any of those time periods might use. In other words, what things on Earth, other than the race's body parts, would make a good basis for widespread common units of measure?
A commenter mentioned Barley Corns, and that is actually the only reference I could find in my research that was actually used historically, and this is exactly the kind of thing I was hoping for in answers. Other items in comments and answers that fit the spirit of the question include horsewidths, trout lengths, birch leave length, ant lengths, bananas, elephant tails, and narwhal tusk. Though horsewidths and barley corns are the only ones that I both know enough about and can confirm are sufficiently consistently sized and widespread.