It's an old game but I found it recently and I'm playing it now.
I got a monster, a Gremlin, which has a fire attack so I gave it to the guy near the bazaar with a grill (after which the grill transformed in a portal). I'd like to get another one but no matter how many times I go against one of its kind and the treats I'm throwing at it, it doesn't want to come with me.
Is it possible to capture two monsters of a kind?