First Houserule for DCC RPG (due to 0-lvl funnel)

GOLDEN LION RULE: (Named for Jad-Bal-Ja in the Tarzan series): For important pets and animal companions, each time the owner levels, the creature gains a new hit die of the same type.  For example, if Girt the 0-lvl herdsman reaches 1st level, Giant his herd dog gains 1d6 hit points as well.

(I used the wolf stats to determine Giant's stats, same hit die, dropped the attack bonus by 1 and the damage die by 1 step.)  

Note that the Judge has final say as to what is a significant animal, but it generally revolves around whether or not the creature is participatory in the game.