-10 was 3.5 edition.
now stuff die at -con or at 0 if they have no con. (like undead\ construct etc).
so the only 'normal animal' that die at -10 is an animal with 10 con.
-Con
Dying
A dying creature is unconscious and near death. Creatures that have negative hit points and have not stabilized are dying. A dying creature can take no actions. On the character’s next turn, after being reduced to negative hit points (but not dead), and on all subsequent turns, the character must make a DC 10 Constitution check to become stable. The character takes a penalty on this roll equal to his negative hit point total. A character that is stable does not need to make this check. A natural 20 on this check is an automatic success. If the character fails this check, he loses 1 hit point. If a dying creature has an amount of negative hit points equal to its Constitution score, it dies.
(Emphasis mine)
So what is an effect? Is it defined anywhere within the rules?
No one is really sure what exactly qualifies as an effect. Certainly, it counts when targeted by spells. But other ways are up for debate, with no official answer.
So does the companion gain darkvision for example?So what is an effect? Is it defined anywhere within the rules?Anything that varies based on creature type.
There's no difference between an animal and a magical beast when it comes to "at what negative HP value does it die?"
No. No new abilities are gained other than what’s listed in the familiar rules.
The common questionable bits are abilities possessed that are based on hit dice. Like a viper’s poison DC is half its hit die+constitution modifier. So does the DC go up with the casters level? There’s no solid answer. For another example, the aether elemental improved familiar is full of abilities based on hit die.