Was there ever a planet between mars and jupiter?
No, unless you count Ceres. It was once popular to think that the asteroid belt was the remains of a planet that got somehow destroyed, but all of the asteroids combined would mass less than Pluto. The current consensus is that the entire Solar System was once something like the asteroid belt, and that while everything else ended up merging into a few planets, the stuff in the belt just never came together to begin with. This is at least partly because Jupiter, in the orbit it’s in, would naturally tend to prevent things from coalescing in that region of space.
Almost certainly not. There was at one time a theory that such a planet existed but was disrupted, with the asteroids as its remnants. This theory has been rejected by virtually every planetological astronomer; only some non-scientists, largely but nto all crackpots, hold it today. (Exception: the ‘Planet V’ hypothesis suggests a possible planet that was thrown into an eccentric orbit about 4 billion years back; the principal justification for this concept is the high incidence of large-object impacts on Earth (and Mars and the Moon) attributed to that time period. It is not considered likely by most competent scholars but at least has more rational support than the discarded and crackpot theories.)
For the record: 1. Phaeton was the name assigned the hypothetical Asteroid Belt planet when that theory had some popular support. 2. Ceres, the largest and first-discovered asteroid, was for a short time thought to be the fifth planet. 3. Opalcat will have to live with a two-point list; there are no other salient points I can think of to make on the topic.
ETA: What Chronos said.
Velikovsky’s theories suggested that there was. 
For 38 years (1807-1845) there were 4 planets between Jupiter and Mars (1 Ceres, 2 Pallas, 3 Juno, and 4 Vesta). It was with the discovery of 5 Astraea that:
So those 4 planets were demoted around 1845.
Lotsa science fiction stories use the idea, because it’s irresistible (Heinlein’s Stranger in a Strange land and James Hogan’s Giants series, to name two examples), but the idea has been discredited for a long time. But it fits so nicely into the Titius-Bode-Blagg-Richardson law.
Ceres is now classified as a dwarf planet.
Cool! Can we travel there and meet them?
