Tynan is the baby you need to save during the side-quest in Hilltop East, in Shadows of Undrentide. Taking the baby for yourself (by saving Tynan and murdering Nora) will result in a bit of an Evil alignment shift. Originally, you could sell him to Szaren as a slave, but Bioware took the script to check for Tynan in your inventory due to negative player feedback.
Keeping Tynan from Shadows of Undrentide and importing the player character into Hordes of the Underdark will allow you to use Tynan in the Beholder Bridge Control Panel. When given the choice, choose "Insert Tynan", and two options will appear. One is a spider and the other is a small humanoid. Choosing one will remove Tynan from your inventory and replace him with Tynan's Top, an item which summons your chosen monster as a permanent summoned party member once per day. Tynan will act like an animal companion as far as unsummoning goes (IE, he will not unsummon upon resting).
The following have been tested using a level 20 PC in HotU Chapter 2.
Tynan the Goblin
Level 1 Humanoid (goblin) Level 18 Barbarian AC 47 216 max HP Str 22 Dex 13 Con 20 Wis 10 Int 11 Cha 8 Alignment Chaotic Neutral Fortitude 18 Reflex 7 Willpower 6 Base Attack Bonus 18
Main Hand Weapon: Handaxe +6
Tynan the Drider
Level 6 Aberration (drider) Level 12 Cleric AC 18 159 max HP Str 15 Dex 15 Con 16 Wis 15 Int 16 Cha 19 Alignment Chaotic Evil Fortitude 13 Reflex 8 Willpower 16 Base Attack Bonus 13
Main Hand Weapon: Mace +6 Offhand Weapon: Mace +6 Third Attack (Drider) 1d4+2
Freedom Effect