Half-Dragon Companion (3.5e Feat)
From D&D Wiki
 Half-Dragon Companion [Special]
You have shown yourself to be an ally of all nature's dragon creatures
Prerequisite: Animal Companion ability, Knowledge(arcana) 2 ranks, Speak Language(Draconic)
Benefit: You may add the Half-Dragon template to any creature you can have as an animal companion. For example with this feat you cannot have a human half-dragon as an animal companion. The level modifier depends on the creature's original challenge rating. Creatures with a challenge rating of one or less have no modifier; other creatures use twice their challenge rating minus one, rounded to the closest multiple of three levels. So creatures with a Challenge Rating of 1 have no modifier, CR 2: level -3, CR 4: level -6, CR 5: level -9, CR 6-7: level -12, CR 8: level -15, etc.