Anything that can spawn in multiple-slots will have an easier time spawning >90%. Nightmares, Najasaurus, Phoenix, Lesser Hiryu, Reptalon, Skree, White Wyrms, etc.
This is because of random stat allocation and where the pet can change at. If a pet will change in the middle of an intensity range, they have an easier time spawning at 90%, even 100%. Take a look at a bell curve to get an idea of stat distribution. The closer a pet is to the middle of the curve (when it changes), the easier it will be to get a favorable stat distribution to give it a higher rating.
A pet that spawns in only 1 slot will have a much harder time being an extremely high rating. You need a near-perfect stat distribution to even hit 90%, which can have an extremely low chance of happening. On the plus side though, it should be equally hard to find an extremely low rated pet (that isn't a multi-slot pet).
Most pets will fall in the 40% - 60% range. I typically advocate that anything 60%+ is a keeper, but it can depend on the pet you are hunting and its spawn range. 70%+ for almost any pet is really good.
As Pawain suggested (*gasps*), if you find a pet and you are unsure if its rating is good, send it to the planner and spec it out to see what it can become. A 60% legacy nightmare and an 80% legacy nightmare can get the same exact spec, one will just have a little higher hit points.