Known Existence: July 2002 (Publish 16, dates vary)
Though a fire steed will always be born with 100.0+ resist, it may or may not stay above 100.0 after taming. A fire steed that requires resurrection will also lose a portion of its 100.0+ resist. None of it can be trained back. All players and npcs (including monsters/animals) have a normal skill limit of 100.0, and cannot train beyond that unless they aquire a new skill limit (such as with a powerscroll).
Note: while the Dev team says this is not a bug, it completely contradicts the line in the Publish 16 taming document that says
Pets will suffer 10% skill loss when they are tamed, but have full potential for training.
Solution: None at this time.