Beast Master
3rd Level Ranger's Companion
Choose a beast the is no larger than Medium and maximum CR of1/4. Add your proficiency bonus to its AC, Attack rolls, and damage rolls as well as saving throws and skills it is proficient in. Its hit points maximum equals its normal maximum or 4 times your Ranger level, which ever is greater.
As you progress in levels, the companions CR rating and size increase as well. It can either be to selecting a new companion or the maturation of the selected companion animal. For example, a wolf eventually "grows" into a Winter wolf.
Level CR Size
3rd Level 1/4 Medium or smaller
6th Level 1/2 Large or smaller
9th Level 1 Large or smaller
13th Level 2 Large or smaller
The companion animal can increase its ability scores with your level of progression. When your companion reaches 6th, 12th and 18th level, you can increase one of it's ability scores by 2, or two of it's ability scores by 1. As normal, you can't increase an ability score above 20 using this feature. Or you can select an appropriate feat, such as Alert, Athlete, Charger, Durable, Grappler, Mobile, Moderately Armored, Observant, Resilient, Sentinel, or Tough.
It can use 1 or more hit dice after a short rest, up to its maximum number of hit dice. For each hit dice spent in this way, roll the die and add your constitution modifier It regains some Hit Dice after a long rest.