Wind Trader Emeritus
Sorry - missed this one. Sea Ponies certainly were possible to catch more than once on the same character, and since they are now cage-able and trade-able, I see no reason why that would have changed.
My understanding of the way the loot system works in MoP is that you will never be shown something you cannot use in any way. So (presumably) in MoP one will never catch a second Sea Turtle after learning the first, because the mount is BoP and effectively unique to your account once learned. Pets should be exempt from this, because they are not uniquely learnt.