A feat typically doesn't care how its prerequisite is met
The Player's Handbook says
Some feats have prerequisites. Your character must have the indicated ability score, class feature, feat, skill, base attack bonus, or other quality designated in order to select or use that feat.…
A character can’t use a feat if he or she has lost a prerequisite. For example, if your character’s Strength drops below 13 because a ray of enfeeblement spell, he or she can’t use the Power Attack feat until the prerequisite is once again met. (87)
Thus, so long as the creature meets the feat's prerequisites when the creature is eligible to take the feat—usually upon gaining a level—the creature can take the feat.
However, when the feat's prerequisites are no longer met, the creature can't realize the feat's benefit even though the creature still possesses the feat. (For example, a creature that can't realize a feat's benefit because the creature's lost one or more prerequisites retains access to the feat, so it meets the prerequisites of other feats that have that "dead" feat as a prerequisite.)
- Yes, the Charisma 11 dragon can cast the spell eagle's splendor to meet the prerequisite of—and therefore be eligible to take—the feat Frightful Presence (Draconomicon 106).
- Once the eagle's splendor spell's duration expires, the dragon can no longer realize the benefits of the feat Frightful Presence unless it somehow again increases its Charisma to 15 or higher.
- It's possible—but highly unlikely—for a creature to have on its character sheet a feat it can't use that was available due to the creature having met the feat's prerequisites between sessions after it gained enough XP to advance to a level in which it was eligible for a new feat. (Also see Class and Level Features on Player's Handbook 59.)
Some DMs are more lenient than others, letting PCs take feats that they can't use all the time. Ask the DM if your Charisma 11 dragon PC can take the feat Frightful Presence anyway, yet only realize the feat's benefit when the dragon PC's Charisma meets the feat's prerequisite. This shouldn't be game-breaking.