It's not חיה but חיית. This is to do with smikhut, construct nouns, which you'll come to later. Literally it is something like "animal of affection". For now just learn these words as they are: חיית מחמד khayat makhmad (pet) and חיות מחמד khayot makhmad (pets).
Pleasant/nice is נחמד nekhmad.