There's no problem with the house pet, it's simply a very rare pet to obtain, probably the rarest in the game. Also, the idea that we'd wipe it from anyones account and make it a mahogany homes exclusive pet is not something we'd ever consider, because there'd be no point in doing that besides outraging players who've gotten it already. We did it with Herbiboar because the rate of the Herbiboar pet was bugged, and it was never meant to be obtainable from herblore. The House pet rates haven't changed at all in the past few years (probably since release), the only thing that changed was that we added the ability to get it from mahogany homes, and trading in curved/long bones. If you want better odds at getting the pet, here are some tips:
1. Have 200m experience in construction - this, as with most other Skilling Pets, will increase your chance to get the house pet by 15x.
2. Use pet drop scrolls.
3. Use a golden hammer - this increases your chances to get the house pet by 25%.
4. Build higher level objects in your home, or do higher level mahogany homes contracts. The multiplier is based on the level of the object you're building, so perhaps unsurprisingly, building higher level objects increases the chance for a pet.
At the end of the day, it is a very rare pet, and you may not get it until you reach 200m exp, or even after, but it is definitely possible to get it.
The reason we don't get into the numbers is because the calculations can get pretty complex and it just makes things more difficult to understand, but I'll try to provide some more information. There are two multiplicative steps, all of the factors "stack", but they're applied at different points in time.
The first multiplicative step is the skill multiplier (we're assuming you're building a piece of furniture here).
The skill variable is determined by the level requirement of what you're building, essentially you'll be between 0.25 and 0.55 for this value, 0.55 being a level 99 piece of furniture and 0.25 being a level 50 piece of furniture.
The golden hammer takes that rate and multiplies it by 25%. (If 0.55, it is now 0.6875)
Then this rate is multiplied by 0.0075, which is the actual house pet rate. (Following the previous value, 0.005156)
Our first multiplicative step is complete, this is the one that's unique to the House pet. The next step below applies to all pets (except boss pets).
Our secondary multiplier variable is 1.25x if you have a pet booster, and a further 10% or 15% for Immortal/Eternal donators. (Up to 1.35x here)
Lastly, if you have 200m XP, that rate is multiplied by 15x.
We can now take the first number we obtained (0.005156) and multiply it by our second multiplier (1.35x or 20.25x). So we either have (0.0069606) for those who don't have 200m XP, or (0.104409) if you do. Those are approximately the best rates for the House pet on a single action (on one single build action build).
The roll is between 0 and 100, so there's a ~1/1000 chance on a build if you have 200m XP and the very best boosts, or 1/15,000 chance with boosts and non-200m XP. Since you can build ~500 pieces of furniture per hour if your clicks are accurate, you have quite a few attempts to roll while training. The pet is meant to be rare, but if you have the proper boosts and 200m XP, it isn't entirely unattainable.