Noah DreamBlood asks about pets, saying:
You can tell normally if its a hybrid or not depending on time it takes to hatch. School pets are 18 hrs i believe hybrids are 24 or so. And i mixed a colossus that had 54 with a stormzilla that was 53 and got another colossus that had 55, whats up with that? lolAha. Unfortunately Friendly happened to give me a question with a subject that I've very little expertise in. However, I can give it my best educated guess. It's what we college kids do best. /bitter chuckle
That and sleeping in class.
The only level 48 pet that I've gotten since the pet updates is the wraith for my Necromancer, and as you said the hatch time was around 18 hours (that or 17 or 11; I get my numbers mixed up sometimes). A good deal of the higher level boss drop pets I've gotten are the same hatch time, lower level ones tend to be less (around 6 hours), and gift card pets even less than that (some of them just 3 minutes). Unfortunately I've never bred one of my own, let alone a hybrid, so I can't confirm your numbers, though I believe you in the matter of their hatch times.
"Umm... little help over here?"
As for the numbers next to their names and how it relates to breeding and such, my first guess was that it represented the number of chromosomes for each breed, which would totally have explained why the baby came out with a new number (new species means a different amount of chromosomes). But according to people on Wizard101Central, the numbers next to your pet name is its pedigree, or in other words the sum of the traits it has already manifested. You know how when you're on your pet summary page in your spellbook, and next to its abilities there are little yellow pips? Well, when all the pips -- full ones and empty -- are added together, that makes the little 54 and 53 next to your parent pets names. As for the number in the parenthases, that is based just on all the full pips -- the yellow ones -- which is why it is substantially less than the first number. There's a fascinating guide for pedigrees HERE.
Now, when you breed pets together, the baby obtains the appearance, class type, and I think even some of the abilities(?) of the parent pet that has the higher stats. Your Colossus had the higher pedigree, but it was 54 and your baby pet had 55. Clearly there is a pattern in there somewhere, although truthfully I haven't the slightest idea what that pattern is. I never really question the all-knowing MCP that is Wizard101, especially when it comes to numbers. I'm kind of numeral phobic. Maybe since you paid a lot of gold for it the game wanted to give you that one extra point. Or maybe there was an added ability from the weaker parent pet. Or maybe they were just going in chronological order (53, 54, 55). It's a complete mystery to me. The most important thing that indicates your pet's abilities is the number in parenthases, as that is a number that rises with much training and care.
The MCP knows all...
If you happen to obtain a lot of gold over the course of your journeys, you should experiment some more, with pets of different schools and stats. I'm especially interested in seeing what people discover about crown pets as they seem like they can probably make some really nifty hybrids.
Fun fact: I wrote the word "pet(s)" 15 times, including the one just now.
Anyways, good luck in your search for the truth, and safe journeys young wizard!