I am designing an encounter with a Giant Owl druid companion involved. I wanted the owl to get the Improved Grapple feat as part of the advancement for being a druid companion. The owl would in no way benefit from Improved Unarmed Strike - it already has natural attacks that are lethal and do not trigger attacks of opportunity as the unarmed strike of a humanoid creature would. By RAW would the owl need Improved Unarmed Strike as a prerequisite for Improved Grapple?
Does a creature with natural attacks need Improved Unarmed Strike to get Improved Grapple?
Oriental Adventures in its chapter on monsters on Feats, in part, says
Certain monster special abilities count as feats for purposes of meeting prerequisites for other feats. A monster with any natural attack form is considered to have the Improved Unarmed Strike feat. A monster with the improved grab special ability is considered to have the Improved Grapple feat. (143 and links added)
This may retroactively explain why the Monster Manual allows its bebilith (42) and its barbed devil (51 and here) to take the Improved Grapple feat despite neither having the Improved Unarmed Strike feat.
However, so far as I'm aware, this information isn't present elsewhere in the entire Dungeons & Dragons, Third Edition (inclusive) corpus, therefore it wasn't subject to the 3.5 revision. With that in mind, a DM may reassess this rule in light of Why a Revision? (Dungeon Master's Guide 4).
In other words, technically, a giant owl (MM 205) meets the prerequisite of the feat Improved Grapple with just its Dex 17 and claw attack—without needing that pesky Improved Unarmed Strike feat—, but if you're a player considering using this rule, ask the DM about it anyway.
1\$\begingroup\$ Pretty sure “I am designing an encounter” suggests that Giorin is the DM. Anyway, +1 for answering the question well, and +bonus for teaching me something new. \$\endgroup\$– KRyanFeb 5 at 21:18
The owl would in no way benefit from Improved Unarmed Strike
Strictly speaking, this isn’t true—a giant owl can make an unarmed strike as appropriate for a creature of its size. It just ordinarily wouldn’t have any desire to do so.
Anyway, no, there is no rule that waives the Improved Unarmed Strike requirement on Improved Grapple for creatures with natural weapons. (Was incorrect on this point: see HeyICanChan’s answer.) Most creatures that are intended to be grappling get it as a bonus feat, or get improved grab, or both.
There are various ways to get Improved Grapple without Improved Unarmed Strike if you take the right class levels—bear-totem barbarian 2nd is probably easiest, though monk 1st is even easier and then you just pick up Improved Unarmed Strike for “free”—but animal companions cannot get class levels. Strictly speaking, I guess, a giant owl could probably get the leg of squid graft from Dragon vol. 318, but at 40,000 gp, that’s pretty rough.
The real answer to this is, if you’re “designing an encounter” as a DM, is to just give the giant owl this feat. Remember,
You can add any sort of spell-like, supernatural, or extraordinary ability to a creature.
(Improving Monsters → Adding Special Abilities)
“A giant owl can ignore the Improved Unarmed Strike prerequisite on Improved Grapple” could be an extraordinary ability of approximately zero value.