[3.5e] Adding friendly creatures to the party

[evnaz]

Recently through a string of weird events some of my players managed to befriend an "Owl, Giant"

So, if this creature stays in the party and manages to gain a level, what should I do to make it stronger?

how do monsters develop?

[5ColouredWalker]

It tells you how to advance them in their stat block.

For a giant owl *Checks SRD*
Normally by HD, but being intelligent and capable of being cohorts you could add class levels if you preferred.

Since it's not a Cohort, I'd treat it as a level 4 and should it level, depending on how lazy I was, I'd ad a level of Magical Beast or a appropriate class level.

[Coidzor]

Well, if you're letting it gain Experience points, you basically either A. Use the Cohort rules, possibly using the Average Party Level for the cohort's master or B. Treat it like a PC that you direct or let players direct but you have final say over.

So you'd need to determine Level Adjustment, unless you're backporting Pathfinder rules for monsters as characters. Or giant Owls have cohort Level Adjustments given and you just apply them to a DM controlled non-cohort.