I've never used csapi, either, but it seems to be the anomaly in Matlab--it operates by row instead of by column as almost everything else does, such as plot here where different columns in the X, Y arrays are treated as different observations.
Anyway, with that in mind it appears you can do what you wish as
t=[30,170,230 ,385]; % indices into x,y arrays wanted
x=xsnake(t,:); % x, y positions array by row for each time
y=ysnake(t,:);
snake=csapi(x,y); % get the spline ppform object for them
xx=linspace(0,3.2,100); % points at which to interpolate spline
plot(xx.',fneval(snake,xx).','-',x.',y.','*')
legend(num2str([1:length(t)].','Snake %d'))
NB: the arrangement by column in plot and the default color cycling with the spline fitted values represented by a solid line and the measured points by the asterisk. The legend will reflect the colors only unless you do some extra work to match the two line styles.
ADDENDUM
Actually, the fixup for the legend isn't bad...save the object handles when creating it and then add the markers. legend ends up drawing eight lines even though they're hidden looking like only four since plot put two sets of four on the axes. The first object handles are the text for the labels, though so the line handles begin at 5.
[~,hO]=legend(num2str([1:length(t)].','Snake %d')); % save object (text,line) handles
set(hO(length(t)+1:2:end),'marker','*') % set the alternate line marker to match
