JoeBlank
Explorer
Can't help on how to play one as a PC, but as far as stats go there is a mouse familiar in Dragon 280:
Mouse: CR -: Fine Animal; HD 1/4d8; hp1; Init +2 (Dex); Spd 15 ft., climb 15 ft.; AC 20 (+8 size, +2 Dex); Atk none; SQ scent; Face 1/2 ft. by 1/2 ft.; Reach 0 ft.; SV Fort +2, Ref +4, Will +1; Str 2, Dex 14, Con 10, Int 2, Wis 12, Cha 3.
Skills: Hide +14, Move Silently +6, Swim +10.
Mouse: CR -: Fine Animal; HD 1/4d8; hp1; Init +2 (Dex); Spd 15 ft., climb 15 ft.; AC 20 (+8 size, +2 Dex); Atk none; SQ scent; Face 1/2 ft. by 1/2 ft.; Reach 0 ft.; SV Fort +2, Ref +4, Will +1; Str 2, Dex 14, Con 10, Int 2, Wis 12, Cha 3.
Skills: Hide +14, Move Silently +6, Swim +10.