[RPG] this longsword that turns into a high-crit chain- or whip-like weapon that I remember

From the 3.5 SRD in the class description for the Cleric:

Weapon and Armor Proficiency

Clerics are proficient with all simple weapons

Which means that normally, Clerics are proficient with simple weapons only. But a Cleric who chooses the War domain becomes proficient in the deity's favored weapon.

A cleric who chooses the War domain receives the Weapon Focus feat related to his deity’s weapon as a bonus feat. He also receives the appropriate Martial Weapon Proficiency feat as a bonus feat, if the weapon falls into that category.

A Cleric without the War domain does not gain that proficiency.

If you can meet the prerequisites of prestige classes by your previous gestalts, than it is reasonable to take Beastmaster as soon as possible. I assume you are going to finish your Convoker advancement, so the character would look like that:

Commoner 6 // Convoker 6 // Beastmaster 1

At the first level Beastmaster provides boost to your effective Druid level, so it will be a companion of 4 level Druid from the spot. I dont realy think you'll very much need any more levels in Beastmaster as it stacks with any companion advancing classes, and they have better class features.

From now on I see two options:

1. Take Druid for spellcastng, if you have room for it in your actions. Five levels will give you access to highest level spells even without that level lost due to Beastmaster. You also will have room for adding more prestige classes at those five levels later, as Druid is a regular class. Finaly, as a regular class, it has no prerequisites. It results in:

   Commoner 6 // Convoker 6 // Beastmaster 1/Druid 5

2. Stack some other prestige and maybe regular classes with nice class features wich progresses your animal companion. I'd consider Wild Plains Outrider, good class with quick advancement and ability to make a full attack after mount's single move. Two remaining levels may be of practically any class wich progresses your animal companion, probably those of a Beastmaster or even some regular class whose class features you like. It results in:

   Commoner 6 // Convoker 6 // Beastmaster 1/Wild Plains Outrider 3/xxx 2

I can't realy wrap my mind around Animal Ally feat mentioned by Chemus. I feel like, depending on your DM's judment, it may be quite usefull, but when to take it is a question I can't answer. Beastmaster and Wild Plains Outrider are both from Complete Adventurer.