Possible Clues:

• Mule, e.g
• Eg cockapoo, grumpy about being confined to bed
• Mix, as dogs
• Mule, say, angry about standing in bottom of river
• Such a strain, irritable and about to get into bed
• Mate (two different species)
• Mongrel
• Angry about bed being put outside for lurcher
• Such a mongrel rottweiler, perhaps, will not be good-tempered