When researching a certain technology, Aztec Monks gain an extra 5 HP which would not normally be awarded.
This makes them particularly resilient.
Other civilizations also provide bonuses to certain unit types, such as Franks who give a 20% HP bonus to cavalry from the Feudal Age, and Vikings and Vietnamese who give HP bonuses to infantry and archers respectively.
However, these bonuses are not linked to any upgrades.
It would be interesting if a civilization gave their infantry bonus HP when researching Scale Mail Armor, Chain Mail Armor, and Plate Mail Armor, but this may not be feasible due to the already existing bonuses from other civilizations.