This page explains how to recruit Poshul in Chrono Cross: The Radical Dreamers Edition.
Poshul is a talking dog (don’t be surprised) that resides in Arni Village, which Serge calls home.
How to Recruit
There are two ways to recruit Poshul.
Firstly, before defeating the Mama Komodo boss at Lizard’s Rock, visit the restaurant near the exit of Arni Village. Go past the waitress and the suspicious barrel and enter the room at the back, up the stairs. You need to open the door to get in.
Inside the room, search under the bed to find a Heckran Bone.
Now make your way to the west side of the village, where Chief Radius is. Near the top, you should see Poshul running around. Approach her and show her the Heckran Bone via the Key Items menu. It can be somewhat fiddly since Poshul won’t stop moving.
If you missed this window, Poshul will automatically join with Leena at Arni Village in Another World after defeating Karsh, Solt and Peppor and declining to recruit Kid. (You can still recruit Kid afterwards, so don’t worry about it.)