It’s got some hippos fighting.
That’s seriously all he can remember, folks, but there can’t be that many sentient warrior hippos in literature, can there? I hope not.
He says it’s really good and I should read it.
How long ago was he a child?
Good Og! Has Spelljammer been around that long already?!
The Happy Hippopotami (1970), by Bill Martin, Jr. With pictures by Bob Velde. Dozens of happy hippopotamuses have a fun-filled holiday at the beach.
George and Martha (1972). Written and illustrated by James Marshall. Relates several episodes in the friendship of two hippopotamuses.
Sequels:
George and Martha Encore (1973).
George and Martha Rise and Shine (1976).
George and Martha, One Fine Day (1978).
Never Babysit the Hippopotamuses! (1993), by Doug Johnson; illustrated by Abby Carter. Babysitting hippopotamuses leads to wacky events and unruly incidents.
I think possibly jayjay’s nailed it - spelljammer is about 15 years or more old (was cancelled in 1993) - and the Gif are the only sentient warrior hippos I can recall.
Maybe one of the Cloakmaster Cycle?