Grandpa likes a Whale but not a Hippo
Grandpa likes Hulk but not Frankenstein
Grandpa likes a Foot but not Eighteen inches
Things he dislikes are linked together by an incredible single long word
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His three dislikes
compose the three parts of the phobia.
Hippopoto --> Hippo
monstro --> Monster (Frankenstein)
sesquipedalio --> Latin literally meaning one and a half feet long
As for his likes:
They are all short words
And as for the hint:
hippopotomonstrosesquipedaliophobia is an incredibly long word.
One rule that fits all the information we have so far is that
Grandpa likes a thing if, and only if, its name doesn't contain the letter I.
In this case
the things he dislikes are linked together by the word "I". Is that word "incredible" and "inconceivable"? Well, for a slightly far-fetched affirmative answer, consider the phrase "I couldn't believe my 'I's" ... :-).
For what it's worth, my guess is that this is not the intended answer. But I think we need rather more information in order to nail down a single right answer. I don't think anything's wrong with the one above, and I bet there are several other answers of approximately equal plausibility.