Through Ranger's Animal Companion class feature grants him the said companion, he lags behind Druid in its efficiency:
This ability functions like the druid ability of the same name, except that the ranger’s effective druid level is one-half his ranger level. A ranger may select from the alternative lists of animal companions just as a druid can, though again his effective druid level is half his ranger level. Like a druid, a ranger cannot select an alternative animal if the choice would reduce his effective druid level below 1st.
This means, among other things, that on his 4th level, Ranger gets an unimproved Animal Companion just like a 1-2nd level Druid. The Owl companion he gets on this level is just a regular Owl (although, it does possess Link and Share Spell abilities).
Getting a Magical Beast companion
If you want a Celestial, or a Giant owl, you'll need an Exalted Companion feat from Book of Exalted Deeds. On 14th Ranger level, you'll be able to get the Giant Owl companion (which is awfully late, but at least it's there)
Since this class feature is so weak, it's recommended either to ask the DM to houserule that the companion gets the Druid's progression (1x your class level instead of 0.5x your class level), or just trade off this class feature for one of the Ranger's Alternate Class Features (like Phynxkin Companion ACF from Dragon Magic), through if you are interested in the roleplay aspect of the class, you're stuck with either the houseruled option, or vanilla version of the Companion