Evolving Companion

At first level you can choose a CR 0 creature to become your familiar, you may also choose a newborn of a higher CR monster, in which case you must choose an appropriate stat block out of another CR 0 creature to use. At every level up the familiar levels with you and you can choose any stat to replace with another one from a 1 rating higher CR monster (your 2 hp CR 0 Crab could now have the 15 hp of a CR 1/8 Camel), or you can choose to grant him 1 trait from a 1 rating higher CR monster (your crab could gain the beast of burden trait from the mule), your familiar can have a maximum of traits equal to 1/4 his level rounded up (1 until level 4, 2 until level 8 etc). Your familiar falls unconscious when at 0 HP and dies on the third failed death Saving throw.