A special frame paladin, it does not matter what it is, normally they would not be eligible to be an animal companion. No matter what the special mount is in another way, when it is summoned to be a paladin mount, it is a magical beast, not an animal.
The Devout Tracker devotee says that your mount is now an animal companion, not if it's, and it's, and that's it. This would normally be impossible, but D & D 3.5e is a system based on exceptions, which means that things like dowries can make exceptions to what would normally be allowed. In this case, the devotee Tracker provides an exception to make your mount, which presumably already exists when you take the feat, and the feat does not suggest limitations on what it may be, in a companion animal.
In short, yes, an unconventional mount can become an animal companion with the Devoted Tracker feat.