Lin Yuan chose the Titan Giant Boa for Zhang Xiaobai and the other three not only because it existed in abundance and could be paired with standard feys, but also because of its large size that made it highly compatible with Zhang Xiaobai's group's Willpower Runes.
Although no one could guarantee the exclusive skills that a fey would gain after it fused with a Willpower Rune and evolved to Fantasy Breed, Lin Yuan had evolved enough feys to Fantasy Breed to understand fey and Willpower Rune compatibility.
The hope Willpower Runes that Zhang Xiaobai's group had comprehended while under great stress was slightly similar to the new life one that Lin Yuan had sensed from plants.
Lin Yuan felt that it was likely for this hope Willpower Rune that was comprehended under great stress to result in the fey it fused with to gain revival exclusive skills.