The paladin mount allows the character to take unconventional creatures as their mounts, such as dragons, magical beasts and things like that.
The druid/ranger animal companion must be an animal.
The Devoted Tracker feat (Complete Adventurer, p. 108) allows you to treat your mount as your animal companion.
Does it give any creature the bonuses, or does the creature need to be eligible be an animal companion to begin with?
A paladin special mount, no matter what it is, would not ordinarily be eligible to be an animal companion. No matter what the special mount would otherwise be, when it is summoned to be a paladin’s mount, it is a magical beast, not an animal.
The Devoted Tracker feat says that your mount is now an animal companion—no if’s, and’s, or but’s. This would normally be impossible—but D&D 3.5e is an exception-based system, which means that things like feats are allowed to make exceptions to what would normally be allowed. In this case, Devoted Tracker is providing an exception to make your mount—which presumably already exists when you take the feat, and the feat suggests no limitations on what it can be—into an animal companion.
In short, yes, an unconventional mount can become an animal companion with the Devoted Tracker feat.
