It was recently suggested to me that the Mammoth Rider's new steeds by default replace your old one.
I argued in return that they are an option, offered to replace your old selections (in case you were stuck with a pony or dog, for instance, due to being small).
Specifically, my argument is that the word "can" is used as opposed to "must." That the line about them replacing your mount later on is specifically to avoid you trying to cheese in multiple combat animals.
I was under the impression due to previous statements that if you have a mount unlocked for yourself from something else, such as due to beast rider or monstrous mount, then it counts as something you can use with your the class features that apply to mounts.
The statement I was looking at in particular is from the PFS FAQ about Paladins which says "You may only select a mount from the listed mounts on page 63 of the Pathfinder RPG Core Rulebook unless another source grants access to additional creature choices."
Edit: Removed a bit of commentary to keep it cleaned up and easy to read.
