"rarest" ? I would think it would be indicated by its drop rate. Due to how the pet spawn works, unborn valks imo would be "rare". Or the qiraji gaurdling/snowy owl, I mean...you only have 90 days out of 365 days a year to get one. Or collectively, a raiding with leashes pet, as each other pet individually has a low drop rate...but you need 10-12 pets to obtain it...so its a time/money sink, making the reward pet a little more rare.
Your criteria on rare is unknown, so...makes it hard to determine.