I am about to start a Pathfinder campaign as a Cavalier for the first time, and I've hit a bit of a quandary. As dictated by Animal Companions, my mount should have one hit die. However, when looking at a boar (my guy is a gnome), the animal has a roll of 2D8+9. Considering Toughness and the boar's +3 CON modifier, does this mean that my boar has 2D8 per hit die (around 18HP), a single D8 per hit die (around 10HP)?
For a cavalier to select a boar, he must be at least 4th level. At 4th level, an animal companion has 4 HD, not 1 (in fact, even at 1st, animal companions still have 2 HD).
The boar, meanwhile, has 2 HD to begin with. Each one is a d8 (all animals use d8 hit dice, and there is no such thing as a “hit die” that consists of multiple dice), so 2d8 total. Monster entries always list HD this way.
Your boar, having 4 HD, will be 4d8 instead of 2d8, plus 4×Con, plus 4 for Toughness. On average, that will be 34 HP.