Exactly what it says on the tin. Real-life T-rex goes up against the Owlbear from D&D. Fight takes place in two rounds and is set in an open forested area around sunrise.
Round 1: Speed not equalized.
Round 2: Speed IS equalized.
Just a few things to consider,
Tyrannosaurus is around 10x heavier than the Owlbear and possesses a bite force of with an absolute maximum of 2,971,641,560 N/m^2 of force (431,000 lbs/in^2) with the use of certain teeth, however the average is around 12,000 lbs/in^2.
With speed not equalized, the Owlbear is, at the very least, 3x faster than the Rex.