Two things I like about this thread:
One: how we’re all just overanalyzing this. The OP’s answer is completely serviceable and, imho, still the best answer of all of the submitted (especially without delving into trivial nit-picking).
Two: the term
is full of win. 
This may help. 
Surely a coyote could be successfully implanted with a dog embryo!
I can think of some other answers, such as:
Humans named John
or…
[spoiler]
Cats with orange stripes…
Assuming that these qualify as ‘names.’ However, they seem to be nearly as good a description of a subset of mammals as mules or ligers. None of them are species, but then, the definitions of a species make it extremely difficult to arrange for repopulation without exempting some of the species population from extinction.[/spoiler]
(My explanation is included with the second sample answer.)
The eggs take less than a year to hatch?
Ex-girlfriends.
Eggs are still alive. If the eggs survived, I wouldn’t say that every platypus had died. Also, young platypusses need to nurse from their mothers. I don’t think many, if any, would survive on their own.
Definitely. I’m still trying to pronounce it.
Also.
Is mule really correct? I thought mammals were supposed to make milk for their offspring. Mules wouldn’t have either, would they? Are they really mammals? I mean, they’re furry and they look like mammals, but they seem to not meet the crucial critera. Like Pluto’s not really a planet, y’know?
Same here. Maybe it should be stated as, “If they all died today, there’d still be some left next year.”
[Female] mules are generally sterile but not always. Wiki seems to imply that fertile females could produce milk.
I’m also quite sure that I’m a mammal, even though I’m male
Um, what? Yes, mules are mammals. I mean, what? Are you sure you’re not confusing them with, say, crocodiles or something?
A possible reading of this would be that you are looking for a species as an answer. But mules aren’t generally considered a species, are they? They’re a hybrid, cross, or some other term.
From the Wiki page:
Wouldn’t any cross meet the criteria as well? You could say the same about a Labradoodle depending on where you draw your dividing lines.
Yeah, I noticed the word “species” didn’t appear in the original clue, so I had a different answer, which IMO is totally valid:
baby humans
And a reversal of the above would also provide another answer to the puzzle making it a riddle with two valid responses.
Great. Now I’ve got Michael Palin in my head telling about how he wants to train crocodiles–you know, cute little fuzzy things with long ears and bobbed tails nibbling on carrots all day?
Platypus?
Nobody but me picked up on this?
Do you know what the creature is called?
Crocodile?
Hinny
There are too many to name. I’d imagine the original was looking for “Mule”, but there’s Hinny, and Zorse just with horses, not to mention the Zonkey. Then there’s Coydogs, Jack-dogs, Coy Wolf, wolf-dog . . . and Geep, and Grizzly-Polar Bear, and Savannah Cats, and Ligers. . .and of course, BAT-BOY!