Could you also say "My least favourite thing" for "Fy nghas beth", to contrast with "Fy hoff beth"?
It isn't accepted. I tried it because "pet hate" isn't something you ever hear in American English.
At present, it should accept 'pet hate', 'the thing I hate the most' and 'pet peeve'.
how should i interpret 'pet hate'? does it mean: 'what I hate most'? I honestly don't think I ever encountered it before...