In monster codex there is:
Gorthek Companions
An orc who takes the Beast Rider feat (Path finder RPG Advanced Race
Guide 56) can choose a gorthek as an animal companion or mount.Starting Stats: Size Medium;
AC +4 natural armor; Speed 40 ft.;
Attack gore (1d8); Ability Scores Str 16, Dex 11,
Con 13, Int 2, Wis 10, Cha 5; Special Qualities
darkvision 60 ft., low-light vision.7th-Level Advancement: Size Large; AC +3
natural armor; Attacks gore (2d6) Ability
Scores Str +8, Dex -2, Con +4; Special
Attacks powerful charge (gore, 4d6 +
twice Strength modifier).
Beast Rider (Orc/Half-Orc)
You gain the service of a monstrous companion or mount.Prerequisites: Animal companion or mount class feature, character level 7th, half-orc or orc.
Benefit: Select one of the following creature types: elephant, pteranodon, rhinoceros, stegosaurus, or triceratops. Add this creature type to your list of possible animal companions or mounts. When summoning a creature of the selected type to serve as a mount or companion, treat your effective druid level as if it were two levels higher (to a maximum of your character level). If the creature is large enough for you to ride, it gains the combat training general purpose (see Handle Animal) at no cost.
Is the bolded part meant as addition to Beast Rider feat or both addition to the feat and limitation for gorthek animal companion usage?
So can druids(and other classes who have no limitations on ac choice) take Gorthek as ac at lvl 1?
A druid needs the beast rider feat to get a Gorthek. Just like a ranger, cavalier, hunter or cleric. However, the feat does not improve a Gorthek, only the elephant, pteranodon, rhinoceros, stegosaurus, and triceratops.
So you are saying for gorthek there is no +2 druid level and no combat training?
If so then it seems it's really not worth taking for anyone.