Maybe they meant the last son of Krypton to actually LEAVE Krypton. 

Under the influence of one particular comet (hey...) that passes by the Earth apparently at irregular intervals, Comet the Super-Horse becomes human for a few days. In this form, he has carried on a on-and-off again romance with Supergirl who is unaware of her handsome stranger's true identity and nature. 

At the time of Crisis #7, someone in the industry joked (I wish I could remember who) that was how Kara died. The rope snapped. 

Beppo was Jor-El's lab monkey and he stowed away aboard Kal-El's rocketship before it was launched into space. He first showed up in a Superbaby story wherein he imitated all of the lil' feller's feats, exaggerating them to super-heroic levels and thus risking Superbaby's secret identity. I believe Superbaby tricked him into going back to live in the jungle as he had been doing since the rocket crashed.

Krypto was the family dog in the El household until Jor-El used him to test an earlier version of the rocket. He programmed the rocket to return immediately but it was sadly knocked off-course by a meteor and drifted through space until it wound up on Earth during the time that Superboy lived in Smallville. 

Streaky was a normal Earth-born house cat (albeit one who thought in complete sentences and had enormous cartoon eyes) who found a piece of X-Kryptonite which gave him super-powers. He kept it rolled up in a ball of string.

Technically, the Legion of Super-Pets also included Chameleon Boy's pets Proty (who sacrificed himself to save Saturn Girl) and Proty II.

Tanthu Flez or "Winged Ones": large white dragons.

Metal Eaters: At least two different varieties showed up over the years.

Thought-Beasts: Triceratop-style quadrupeds with large screens built into their skulls that displayed what they were thinking about; usually it involved running you down and eating you.

Kryptonian Babootch: A lil' kaola bear looking thing that Superman once said was included in his favorite recipe of his mother's. "But if you have a couple of pork chops, you can fake it..." he told Lois. One would hope so! Those things looked adorable!

Flame Dragon: Fire-breathing monster with a full complement of super-powers whenever one shows up on Earth. Superman has gotten rid of them by taking away their powers with Red K and leaving them frozen alive out beyond the orbit of Pluto and by dumping one into the prehistoric past. Yikes... hope it doesn't squash any butterflies while it's back there...

Once upon a time, comics were about imagination and not wrist-slapping and the imposition of arbitrary restrictions.

***

Not officially. Clearly, the Superman stories from the Mort Weisinger years contradict what was canonical through the very early '50s. But there was never a "hard" changeover point, especially since the whole idea of Earths One and Two didn't appear until 1961.

And with Batman and Robin there's really no "hard" changeover point. Especially since Robin didn't visibly age until the late 1960s. It's not like he got older and then got younger again. 

There are only a few things that separate the "Golden Age Batman" era from the "Silver Age" one within the pages of BATMAN and DETECTIVE COMICS. Two-Face was Harvey Kent before he was Harvey Dent, for example. I think a few early Batman and Robin stories involved Nazi spies; clearly, not canonical for the Batman and Robin of the mid-50s, who looked no older than they did during WWII. 

(I always had a bit of a problem with the Earth-Two Superman. He wasn't really the Superman of the '40s. He never stopped working for George Taylor at the Daily Star, when in the actual comics the changeover to Perry White at the Daily Planet happened in 1940!)