I believe there's specific text in magebred that says you can't.
I checked three times and seen nothing of the sort. So behold the pwnage.
Look again.
ECS, p. 37:
A druid or ranger may never acquire a magebred animal as a companion. Most druids and rangers are ambivalent toward such creatures, while some consider magebred animals to be corruptions of nature.
Then again, Five Nations, p. 75 changes this to:
A druid from Breland of 7th level or higher or a ranger from Breland of 14th level or higher may select a magebred brown bear as her animal companion instead of a polar bear.
A druid from Breland of 7th level or higher or a ranger from Breland of 14th level or higher may select a magebred ghost tiger as her animal companion instead of a dire lion.